I know very little, if anything, about builds. However, I can throw out RP concepts and if you wish to play an arcane caster, bards and warlocks are viable--if you get the proper feats to wear mithral FP without any penalty. Else, I know barbarians to wear medium armor. A cleric may be an option, but I don't know about the build's viability for the maximum DEX bonus (+3 if I remember correctly, so you would need a 12 base and +4 DEX item or spell).
Wizzies and sorcs, unless you're feeling a little dangerous, I wouldn't really recommend it. If you wish to play a fighter-type character, perhaps dual-wielding rapiers with finesse to meet the DEX requirement for maximum AC bonus?
I honestly have no clue and am spitballing.
For the tiefling:
1. The character may well have been an escaped slave from another city or a slave from S'shamath who has gotten their hands onto a spellbook and has learned the Art. The most difficult part for this concept, I believe, is explaining away the escape itself; to drow, slaves are something that can be worked to death and then simply replaced. How would this character have survived that, then the journey to S'shamath?
2. Perhaps this tiefling was the child of a surface exile or slave. Perhaps the child of a merchant, considering that S'shamath is a trading hub. It's difficult for a tiefling because there would be very few free humans in drow society, at least, and I don't quite anticipate a surface exile to make it alone or to even choose the UD. There's Luskan, Roaringshore, and the like, for crying out loud!
Rukky boi:
1. As above, escaped slave. You could also be a bit of muscle--whichever archetype you choose--for a Darkwoods gang that was taken out by another.
2. You're hired muscle, a mercenary. Who wouldn't want a fiend-blooded orc behind them during a meeting (although they are tiny and adorable).
3. Just all of this.
"In general the tanarukks looked down upon any creatures they considered weaker than themselves and held a special enmity for elves. The only creatures they respected were demons and powerful fiend-touched beings, such as half-fiends. They viewed orcs and half-orcs as valuable allies, but did not consider them equals and would often coerce them into breeding with other tanarukk. Most other intelligent humanoid races that came across tanarukks were likely to be eaten and tortured, more often than not eaten alive." They are also far less patriarchal than normal orcs, so keep that in mind. However, from an RP perspective, this is both interesting and challenging.
Duergar:
1. Duergar will come to the city to trade. There is some clan embassy in Undrek Naudal, if I remember correctly. You can be a trader yourself or guard a caravan. It may have been destroyed along the journey, right when your character was ALMOST there. Maybe it got there. Maybe they left your character behind.
2. Duergar exile.
"Because of their pessimism, duergar rarely adventured of their own free will and were instead most commonly exiles cast out of their society." These exiles are also found with facial and arm tattoos after committing petty crime. These mark them as traitors to their people.
3. I have no idea what this is but check it out:
"A few duergar turned away from the worship of the wicked gods who ruled over the duergar such as Laduguer and Deep Duerra and found salvation through the Morndinsamman."
Svirfy:
1. If you want an RP-heavy svirf, you'll most likely stick around Rockrun; they're suspicious little things and are wary of drow especially.
2.
"Although far more serious than most gnomes, deep gnomes nonetheless exhibited the same insatiable curiosity and craftiness when put in the right circumstances. It was this trait, more than any other, which led some deep gnomes to abandon their cautious upbringing and explore the world around them. Some of these wandering deep gnomes turned upwards and investigated the surface world from which their ancestors came, particularly deep gnome illusionists who hoped to find advanced instruction in the Art beyond what could be obtained in their isolated homes. Others become prospectors, searching the Underdark for new veins of gems and ore to mine." A svirf prospector honestly sounds extremely fun.