I was thinking about this too, so here's my build (added an additional Feat from your houserules):
Lvl 4 Tiefling, Archfey Tome Warlock
This build points to forgive the melee and AC options for a more focused caster. I liked your idea of an hybrid, but seems to me that it really needs to be an hexblade, going in the direction of the other classical builds around. Tiefling adds some interesting features, while allowing to reach 20 CHA on lvl 4, increasing the overall effectiveness and chance to hit of the few spells you can cast.
By Lvl 4 you have:
- Cantrips: Eldritch Blast, Minor Illusion, Vicious Mockery, Thorn Whip, Shocking Grasp, Thaumaturgy, Friends
- Lvl 1 Spells: Hex, Sleep, Faerie Fire
- Lvl 2 Spells: Hold Person, Phantasmal Force
- Hellish Rebuke 1\day (2d10)
- Fire Resistance
- Agonizing and Repelling Blast
I chose Archfey over Old One for several reasons:
- Spells: Dissonant Whispers and Tasha's seems kinda redundant to me, especially when you can get Hold Person at lvl 4. Sleep it's an
excellent spell vs. low level mass enemies, it has no concentration
and grows with spell slots. Faerie Fire can grant advantage to the
entire party vs one or more hard targets. Both pacts have Phantasmal
Force which is a great disable for low int enemies and fun to use too.
- Pact Powers: Fey Presence is an area disable which fits more the theme than Telepathy granted by Old One. At lvl6 Entropic ward is
fine, but Misty Escape is simply awesome.
Invocations: I'd go to PimpMyBlast just because this is your only reliable source of damage in a lot of situations where CC or forced movement are not options - If all CCs fails, if you want to blast that minion in the backrows not worthy a spell, in long fights, ecc..
But if you like, you can forgo one of the blast invocations to get rituals, which can give you a bit more flexibility and a Familiar.
Feats: i'd really like going first at all costs with this build, so i'll take Alert. A well placed Sleep or Hold person the first round is no joke. Otherwise Moderately Armored can give you better ac with a shield, but going melee will require War Caster too to be a viable option.
Ability scores in order of importance: Max Cha, High Dex, High Con, Mid Wis, Low Int, Low Str
Role: cc\secondary caster
Playstyle: Disable high treat enemies as soon as you can. Use forced movement (Blast, Thorn Whip) to generate AO from allies and reposition enemies at your vantage. Minor Illusion to modify battlefield (covers, false gaps, ecc), Vicious Mockery to help ally, Shocking Grasp to disengage, Thaumaturgy\Friends as social tool.
Progression: At lvl5 you can get the infamous Devil's Sight + Darkness combo which can transform you into a walking nightmare of disabling madness, very hard to target. Lots of tactics here, disabling sight with plain movement is a powerful option, plus you attack with advantage; just take care of LOS of your allies. If you want more utility you can always get Rituals. Darkness is 1/day from Tiefling so you can also get a lvl 3 spell. While patron's spells are not that great, at lvl 3 you have 2 strong area disables (Fear and Hypnotic Pattern) and Counterspell, which is really useful if you are the only caster in the party.
Here is Lvl detailed progression:
Tiefling: +2 Cha, +1 Int, Thaumaturgy
Patron Fey: 1-turn Area disable
Spells: 0 - Eldritch Blast, Minor Illusion; 1 - Hex, Sleep
Invocations: Agonizing + Repelling
Spells: 1 - Faerie Fire
Pact of the Tome: Vicious Mockery, Thorn Whip, Shocking Grasp
Spells: 2 - Phantasmal Force
Tiefling: Hellish Rebuke 1/day
Spells: 0 - Friends; 2 - Hold Person
Feats: Alert (from houserule 4th lvl)
Charmed is a condition, described in the Player's Handbook. The rules for that condition (and only the rules for that condition, if you're going for strict RAW) apply to anyone affected by Create Thrall.
It's also common for the charmed condition to have additional story effects. Often, unless the charmed character has immediate reason to resist, they'll be amenable to reasonable suggestions, though this at least is what you should be able to rely on even in the strictest of situations:
From the D&D Basic Rules (and the Player's Handbook differs very little):
A charmed creature can't attack the charmer or target the charmer with harmful abilities or magical effects.
The charmer has advantage on any ability check to interact socially with the creature.
You could try talking to them. Certainly, there are magical options that allow you to forcibly change the alignment of a creature, but the good old fashioned method of explaining to a villain the error of their ways is always an option.
(Admittedly, it's a high-maintenance option that's unlikely to succeed without spending a vast amount of time and effort to get the villain to sit still and think and explain their motivations and their formative childhood experiences and the way they think about ethics, so that you can take that information and use it as the basis for a very persuasive series of discussions of your own worldview and how various "good" ethical and moral stances are superior (both for the individual and the group) even within the villain's own view of the world... But then, if it was easy, they'd just ask mad wizards to quit necromancy, instead of hiring paladins to deal with the problem.)
Regardless of method, redeeming villains through play will only be possible with your GM's consent, and that might be harder to obtain than you suspect; there are a variety of reasons why a GM might want to make villains irredeemable or not focus on redemption as a part of gameplay.