Agree that Celestial Warlock would make a great addition. Mechanically, they wouldn't require a lot of extra coding but they would add a lot of flavor.
I don’t think a good aligned warlock who makes a literal deal with a devil due to extreme circumstances is beyond reason. It’s certainly not a great idea, but as a last resort desperate gamble I find it rather a compelling situation for role playing.
You are ignoring a huge part of warlock. Their pact is not just a "deal". They need to link their souls with the patron, get infused with a certain type of energy and a apprentice of that patron. It is a thing which changes your body, soul and mind. Not just a "puff i gained a patron".
That said, on D&D, Good/Evil, Law/Chaos aren't just concepts.
Are forces like electric energy. There are items that can only be equipped by evil, spells which deals increased damage on evil creatures, and many summons has restrictions. A Good guy can't even summon a demon, a Demon capable of shaping a human soul and teaching eldritch arcane knowledge to a mortal is far above what even summon monster IX on 3.5e can do. And even if he summon that demon and does the ritual right. His soul would be infused with so many energy from the negative planes that he will become evil. At the same way that a Lurker of The deep Patron can't teach fire ball and other fire spells to his apprentices, a Demon can't even link his soul to a lawful good guy. In D&D world, fire and water are opposite forces, like good and evil, law and chaos.
http://www.d20srd.org/srd/spells/summonMonsterIX.htmAnd even on 2e, which introduced Witch/Warlock, can only has his ties to his/her patron severed off, with the strongest spell that can be casted and it affects his/her power.
The Complete Wizard's Handbook(2e) - PAGE 65
"The Witch kit cannot be abandoned. If a Witch manages to sever all ties with the entities responsible for her instruction (usually requiring the power of a wish or its equivalent), she loses two experience levels. (...)If is needed a wish to separate a warlock from his master, is needed at least a wish to link then.
Every warlock patron should be at least CR 30+ on 2e and at least CR 22+ on 5e ---------------------------
That said, i don't have hopes for Lurker of the Deep warlock on BG3 BUT there are a thing called modding. I hope to see UA content on a DLC or mod...