No, it is not balanced

Warlock sub classes tend to be quite powerful, as a rule. So it's understandable you would want to give your class some major benefits. However, there are several balance problems with this class.

Access to overleveled spells

Two of your four class features above are dedicated to the daily casting of a spell. Given Warlock's limited abilities to cast spells above 5th level, this is a reasonable idea for a class feature. However, in both cases you've given the warlock access to a spell several levels before another caster would get them.

A full caster (e.g. Wizard) would normally have access to 7th level spells at 13th level, and 8th level spells at 15th level. But you've given this class access to the 7th level spell Plane Shift at 10th level, and the 8th level spell Maze at 14th level.

The spells in this game are balanced around the assumption that characters get access to them at the appropriate levels. In fact, your version of Plane Shift is considerably more powerful than the usual version, since you do not require a different 250 gp focus for each plane. So it's likely that these earlier access to superior spells might cause some balance issues in play.

I would recommend allowing the warlock to cast Plane Shift as described as its 14th level feature, and replace the 10th level feature with something else. That might be more balanced compared to other classes.

Resource-free healing

As it is currently worded, your "severance" feature ensures that after any combat, you can quickly heal yourself up to half hit points without spending any resources (other than bonus actions), unless you've lost a limb. This is an extremely powerful ability, that essentially means you will always be at half or higher hit points after every combat.

The ability to regain a limb is also quite powerful. Although many DMs don't include the possibility of limb loss, it is a major impediment to a character if it happens. The lowest cost way to heal it normally is the 7th level spell Regeneration, which usually could only be cast at 13th level. To give a character access to this at 6th level is very likely to cause balance issues.

If the ability to regain 1d4+Cha hit points was limited to once a short rest, or took an action and only gave you temporary hit points, and the ability to reattach a limb was removed until 14th level, this feature might be more balanced.

Your expanded spell list may be overpowered

Haste is one of the best buff spells in the game. The ability to cast it twice per short rest at 5th level may easily be unbalancing on its own. And the ability to cast Death Ward multiple times throughout the day (essentially spending any unspent spell slots on it for another party member whenever you are about to take a short rest) is a serious balance issue on its own. It might be ok to have one of these spells on your expanded spell list (although either would be a campaign defining class feature), but I'd heavily recommend against having them both.

Serenity's Shadow is problematic when combined with other characters

The ability to have an "always on" advantage to stealth checks is quite powerful to begin with. Add to that a +1 or +2 to AC at all levels, and you've got a pretty sizable advantage over some other classes. But these features in and of themselves are unlikely to unbalance your character, and is more or less in line with what other warlock classes get at first level (especially front loaded ones like the Hexblade).

These features may cause some issues when multiclassing (Rogues and Paladins would both jump at the opportunity to have these "always on" abilities). However, that isn't necessarily out of balance with established classes either (again, compared to the Hexblade).

What makes Serenity's Shadow problematic is the ability to inflict vulnerability to "slashing, bludgeoning, and piercing damage until the end of your turn." Compare this to the Grave Cleric's chanel divinity ability: this gives vulnerability to one attack, and it costs the Cleric's action to use. Your ability will work for an unlimited number of attacks, and lets the warlock do a normal attack in order to use it.

As a simple example of how this could be abused, consider a team with a Necromancer Wizard, a Rogue, and your Lady of Pain Warlock. The Wizard could instruct all of its skelletons to use the Ready action and fire at your enemy when you attack it. Similarly, the Rogue could Ready an action to fire with the same trigger. All of these attacks would then deal double damage to the target (since the imposition of vulnerability has no saving throw and does not require the Warlock's attack to hit), essentially giving most of your team a free turn due to your one ability.

This ability wouldn't be game breaking for a lot of parties. After all, at higher levels most martial characters (who are the ones who usually deal the most bludgeoning, piercing, and slashing damage) do more damage on their own turns than double the damage of one attack, so it would be a mistake for them to Ready actions and attack once on your turn. But this ability is potentially encounter ending enough that an entire party might design their characters based entirely around it. I would not be at all surprised to find this happening.

Don't give up! You're off to an intriguing start

A lot of your proposed features are extremely interesting. I especially like how you have them all tied around coherent central themes. But I think most if not all of the features proposed here need some rebalancing before it's a fully balanced Patron.

This answer is going to come off as extremely negative. Unfortunately that is my best analysis of the class. So before I get into I want to say; I love the theme of what you were going for. A patron that could be a level 20 character from a previous campaign is great. I also don't want to discourage you from homebrewing stuff in general, this class has a lot of issues though and I felt it was necessary to point them out.

This is unbalanced and breaks the style of warlock patrons

This patron makes such a departure from the typical structure of a warlock patron that it is somewhat difficult to assess in balance terms. My gut says it is very over-powered and I will try to go through each feature to explain why. Note; this analysis is focused on comparing it to other patrons not the entire warlock class.

Expanded Spell List

A standard feature of warlock patrons that you have re-interpreted, both in good and bad ways. Overall your version is overpowered and needs to be fixed.

Spell List As Spells Known

The PHB patrons get access to 2 spells of each level from 1st to 5th. They are simply added to the spell list and do not add to spells known. The celestial and hexblade patrons from XGtE only get access to 1 spell of each level but they still do not count as known spells.

Expanding the spells known of the warlock is completely against the design of all other patrons. This part of the feature is completely broken and needs to be removed.

@illustro points out that as worded this does not add additional spells known but instead forces the warlock to learn those spells. If this is the case then this feature is actually extremely weak, far worse than any of the other patron spell list options.

Choice of spell list

The choice of three different lists based on the spellcasting class of the patron is a cool thematic choice. However all three lists contain spells that are already available to the base warlock. The warlock list is worse than useless, while major image from the wizard list and banishment from the sorcerer list are already available to warlocks.

I would like to see this feature remain but it needs work on the spells chosen. I would drop the warlock list and a add bard, and/or druid list instead.

Lesser Mystic Arcanum

Nope. This feature is entirely broken. At 9th level your warlock has an additional 5 spell slots per day above and beyond their typical 3 per short rest. Not all warlock spells benefit from up-casting and having them in lower level slots does not sufficiently reduce their power. Giving this many additional spells is way, way stronger than anything any of the other patrons give at this level or at any level.

There is no point even comparing this to the other patrons in detail. In summary it is very overpowered and needs to be removed entirely.

Strategic Spell

This one isn't too bad. Rogues and monks already have access to this feature. However it does go against the normal style of the warlock. Apart from the celestial patron every other patron grants a once per short rest ability not an always on feature. By not being limited to pact slots this feature works on cantrips and is therefore infinite.

To answer your follow up questions, removing Dash makes no real difference, Disengage and Dash are about equal. And no; you absolutely should not include Dodge. Dodge is way stronger than either of the others, there is a reason rogue's cunning action does not include it.

This feature is likely a little overpowered due to unlimited uses, but only when compared to the other warlock patrons. Overall this feature is not game-breaking given you should be at range anyway.

Eldritch Weave

At first this one seems ok, but it is likely overpowered. Gaining temporary hitpoints is very similar to the Celestial Resistance feature of that patron, though that one allows you to also protect other creatures.

The issue with this feature is the additional feature: "your concentration cannot be broken until you lose all of these temporary hit points". This is extremely strong. Spellcasters typically take entire feats (War Caster, Resilient (Con)) just to buff their concentration checks. Your feature is negating them entire for a minute.

Yes, this benefit ends early if you get hit enough. But, at a minimum of 20HP and increasing every level this will take a few rounds. Assuming you are concentrating on some disabling spell like hold person or similar it might be impossible for the enemy to deal enough damage to break this before your party takes out your target.

This feature is overpowered and should be changed.

Greater Mystic Arcanum

If the early version of this feature wasn't so utterly unbalanced this would almost be fine. However; it isn't, and neither is this. At will hex at 14th level, then suggestion at 18th level is very powerful.

In fact this feature is exactly eqivilent to the 18th level Spell Mastery feature of the wizard, barring some flexibility. No other patron gets a feature anything like this. Several only get a single once per long rest ability.

At will casting of leveled spells is normally restricted to Eldritch Invocations. Allowing it in addition to those completely breaks the style and design of the Warlock. This is utterly unbalanced and needs to be removed.

Conclusion

Scrap the whole thing. The features are almost all overpowered and don't suit the style of the Warlock at all. Any one of the features (bar Strategic Spell) would be enough to make this class overpowered. Added together this is completely busted.

You are adding flexibility by increasing spells known with your 1st level features, and you are increasing overall power level by giving out way more spell slots. This Warlock is so much better than the others there is almost no reason to look at them unless you want to be a melee fighter.

Back to the drawing board for this one.