DND_Reborn
The High Aldwin
Perhaps you are unfamiliar with the darkness spell, especially this bit "A creature with darkvision can’t see through this darkness, and nonmagical light can’t illuminate it." I should not need to explain how that combines with this "You can see normally in darkness, both magical and nonmagical". Yea there are indeed dozens, like for example how level 19 sorcerer with one level of warlock does not get back 4x 1st, 2x 2nd-5th, 2x 6th-7th, & 1x8th-9th level sorcerer granted slots +4x 5th level warlock pact magic slots just because wotc can't be bothered to errata "You regain all expended spell slots when you finish a short or long rest." leaving GM's to stop the game and point that out over & over again even to players using ddb. Wotc's refusal to errata devils sight too is just a cherry on top of the nepotism* sandwich that is warlock dip.
* Since you do so dislike terms like playwright's/director's/GM's girlfriend. Or perhaps you's prefer it if it was simply the awkwardly worded "GM's Significant Other".
You really need to get yourself focused.
First, what do you need to explain about the spell darkness and the use of DS? Darkness specifies how DV is affected while DS specifically states you can see "normally" in darkness, even magical darkness. That is pretty straightforward, so where is your confusion?
Now you're jumping into spell slot recovery. First, your example is erroneous. A 19th level Sorcerer would have 4/ 3/ 3/ 3/ 3/ 2/ 1/ 1/ 1 spells slots as a Sorcerer caster plus 1 1st level slot as a Warlock. You have an extra 7th level slot and way too many warlock slots. The slots are kept track of separately as per the section on multiclassing. Sorcerer slots recover on a long rest, as normal. The single Warlock slot recovers on a short or long rest, again as normal. So, it makes sense to always use the warlock slot first when you cast a 1st level spell, since you can recover that slot on either rest. Again, where is your confusion? What exactly are DM's having to explain over and over again?
Regardless of whatever you want to call it (whatever "it" is anyway...), what is your issue with the warlock dip? If you don't like it, don't play with multiclassing. And what that has anything to do with DS and it being unbalanced is not being addressed.
One last time, what is unbalanced about DS?