Well, if you don't mind making a pact with some powerful entity, you could get 2 levels of warlock. This will give you access to a few spells, some of which perhaps being interesting (both stats and flavor-wise) for your character. But most of all, this will give you access to 2 invocations.
One of these invocations should be Devil's Sight, which grants you the ability to see through any form of darkness, magical or otherwise, for up to 120 feet. This is more than enough to cover the 15 feet of the darkness spell. Cast it on your clothes, and bring terror to your enemies.
While this is a good strategy (generally favored by blade pact warlocks), it has some limitations. While under the effect of the darkness spell, your allies cannot see you. As such, any spell requiring line of sight that they would like to cast on you simply cannot work. Healing you might be problematic, as it requires your allies to be able to touch you. They might know that you are smack in the middle of that sphere of pure black darkness but, once they get inside, it's suddenly not so easy to find you anymore.
These limitations also affect your enemies, however. Any attack from the outside of the sphere should have disadvantage as your enemies, even if they can approximate your position, can't exactly see you to aim properly. Enemies inside the darkness should also suffer from disadvantage when attacking you, unless they possess some form of blindsight or a similar ability. When you attack them, you should have advantage on the roll since you are functionally invisible for them.
The biggest cost of this strategy is the fact that it requires multi-classing. While the lvl 19 and 20 abilities for monk would not really be missed (realistically, few campaigns will reach these levels), it will still set you back 2 levels in obtaining you other core monk powers. Note that doing this would get you 2 invocations, however, and that some of them can be quite interesting for a ninja-esque character:
Armor of Shadows (cast mage armor at will, which might be better than
using your wisdom for AC if its under 16),
Eldritch Sight (cast detect magic at will),
- Eyes of the Runekeeper (allows you to read anything, even languages
you don't know... useful for spying!),
- Gaze of Two Minds (share the eyes of a willing target, more spying!),
- Mask of Many Faces (disguise self at will!), etc...
TL;DR
Sacrifice 2 monk levels and multiclass as a warlock. Gain some minor but potentially useful spellcasting, as well as the ability to see through magical darkness and one more ability of your choice!
Breaking your overall question down you ask:
Would someone with darkvision or Devil's Sight be able to discern those lighting differences we consider important to our normal vision?
Would someone with darkvision or Devil's Sight be able to detect where subtle shadows are when there's a faint light source to cast them?
Do the rules have anything to say, or is there guidance from closely connected material or other editions available?
Darkvision
A discussion of the difference between these the rules describing darkvision can be found here what is the correct interpretation of darkvision and the rest of this answer will assume that darkvision is consistently applied as written in the race descriptions (e.g. dwarf PHB p.20) though it actually does not make much difference to the answers.
Darkvision PHB p.20:
You can see in dim light within 60 feet of you as if it were bright light, and in darkness as if it were dim light. You can’t discern color in darkness, only shades of gray.
There are two specific levels of light covered here:
- Dim light, where there is a source of light
- Darkness, where there is no source of light
In both cases darkvision enhances vision such that "you see as if it were [the next higher level of lighting condition]". Obviously you can also see at all light levels in between using the same "as if" rule and there can be no shadows if there is no light source. As always the light level present are adjudged by the DM.
Addressing your questions one by one:
1. Would someone with darkvision be able to discern those lighting differences we consider important to our normal vision?
Using the "as if" rule YES, with the exception that colour is replaced with shades of grey in darkness. If a creature with normal vision would be able to discern the lighting difference in the "as if" lighting condition, then so can a creature with darkvision in the actual lighting condition. Otherwise they can't.
2. Would someone with darkvision be able to detect where subtle shadows are when there's a faint light source to cast them?
The question appears to be asking "would you be able to see subtle shadows if the lighting conditions were less than dim but more than darkness?"
If the DM adjudges that a creature with normal vision in the "as if" light level would be able to see the subtle shadows in question then YES, a creature with darkvision can see them in the actual lighting conditions. There is obviously no sudden jump between darkvision's benefits in dim light to those in darkness, it is continuous.
Devil's Sight
There is a discussion about how the Warlock ability Devil's Sight works here.
Warlock's Devil's Sight (PHB p108):
You can see normally in darkness, both magical and nonmagical, to a distance of 120 feet.
This lets you see in total darkness as if it were full light. You gain no bonuses in dim light from Devil's Sight so the answer for darkvision above applies. However for full darkness you get "normal" sight. So it is full colour and any enhancements (goggles of minute seeing) also work. However as there is no light source there will be no shadows. Weird.
Addressing your questions one by one:
1. Would someone with Warlock's Devil's Sight be able to discern those lighting differences we consider important to our normal vision?
In dim light no. In total darkness yes, with the caveat that there would be no shadows. Darkvision is a separate ability.
2. Would someone with Warlock's Devil's Sight be able to detect where subtle shadows are when there's a faint light source to cast them?
No, it has no effect in dim light, only in darkness. Darkvision is a separate ability.
A devil's Devil's Sight works differently, however. For instance the Barbed Devil (MM p.70):
Magical darkness doesn't impede the devil's darkvision
So the answers for darkvision above would apply, with the caveat that magical darkness does not impede the darkvision.
Darkvision and Devil's Sight
3. Do the rules have anything to say, or is there guidance from closely connected material or other editions available?
The RAW from the 5e core rule books actually cover the questions you ask. I can find no other guidance specific to your questions in 5e supplementary material. As stated elsewhere, rules from other editions are not a reliable guide.
To list the sources defining darkvision that I have found in the core rule books:
- PHB p.183 Darkvision
- PHB race darkvision descriptions (e.g. p.20 dwarf)
- Monster Manual p.9 "monster" specific darkvision
- PHB p.108 Warlock's Devil's Sight
- Monster Manual p.70 Barbed Devil (and other devils) Devil's Sight
- (Sage Advice on Devil's Sight)[http://www.sageadvice.eu/2015/11/18/devils-sight-vision/]
And there are also rpg stack exchange questions on this, as referenced above:
- what is the correct interpretation of darkvision
- how should a warlock's Devil's Sight work
Best Answer
Yes, it is possible but...
The number of ways you can go about it is limited to somehow seeing through magical darkness like by getting Truesight.
Or by picking up 2 levels of Warlock for the Devil's Sight Eldritch Invocation, in order for you to see through magical darkness up to 120 feet.
Or by any means like the 6th-level Divination Spell True Seeing, or a Gem of Seeing (page 172 of the DMG) also does the trick, or you can homebrew a magic item that serves this purpose.
What is important is that you can see the area of magical darkness/dim light created, all others might still see it as magical darkness, that doesn't matter.
However, the bottom line is that both of these features were not meant to provide a mini-portal for the monk to Shadow Step through because Shadow Step, by its wording:
Emphasis on shadow here, we can see that the intent was for the Monk to step through shadows, not magical darkness. I know this sounds silly but if we look at the next emphasis, "you can see", it affirms this intent by limiting the area a Monk can step through to an area he can see. Hence, these two were not meant to be used in-tandem due to its incompatible properties. However, there are many uses for Darkness and Shadow Step alone in themselves. Darkness can be used to Blind enemies, causing them to have Disadvantage on Attack Rolls. One clever trick is also to use Darkness to cover a room's light source like a single torch, Darkness negates that source and engulfs the whole room in natural darkness, giving any creature with Darkvision advantage. Shadow Step can enable the Monk to avoid stepping into a brightly-lit portion of a room (and thereby avoid being seen) by teleporting to a dimly-lit portion.
Don't misunderstand me, stepping through magical darkness can be done but you'll have to work for it