BG2 not big on respawning dudes at least (and you like never need to revisit a map outside of Athkatla anyway).
Durlag's Tower is indeed TotSC content and yes it is indeed ridiculously profitable and shovels EXP at you by the bucket load.
Watcher's Keep is available from the start of BG2 with ToB installed and is kind of Durlag's Tower parallel. All the other content is locked until ToB itself. Doing Watcher's Keep straight out of Irenicus' dungeon is ill advised and great fun if you know how to game the system (would be done to try and get stat ups early on level 3 or so of the dungeon and of course the shiny +5s that ToB showers you in). Only downside to doing it early it is that for some stupid reason the first tier of enemy level scaling prevents a guy spawning with one of the upgrade items for the shinies on the first floor (upgrades only possible in ToB itself, so no +6s in SoA, you will just have to put up with +4 and +5 broke shit).
It is for a Bastard Sword though , so it isn't really a big deal, its just a bit short sighted design on their loot tables (and I don't know of a mod that actually fixes it, for all that it should be super easy fix). No big deal because Bastard Swords suck anyway, the only other good one outside of generic ones is like a +3 you get in the Underdark from a shop.
Maybe they should have been left rolled into Longsword proficiency, but those didn't need more support (maybe two-handers? I don't think that would work so great, but it could be a neat little niche to be get 2 handed weapon prof with a sword/shield or both weapon profs or something at the cost of your weapons suuuuuucking).
Anyway TotSC content is pretty fucking nuts (that Greater Werewolf Fen talked about earlier is also some of that) and super super skippable. I legit mostly just go to Ulgoth's beard to buy spells and a cloak, loot top level of Durlag's tower and don't touch it these days.
Edit - Actually there seems to be some dispute over level effecting the spawning of the status with the Foebane on it, saying that it will always spawn. That seems counter to a lot of user feedback (my memory of the sequence is the same) where it didn't spawn no matter how you completed the event leading into it and people using patch that forces max level enemy spawns no matter what saying that it fixed the issue. Not really sure.
Edit 2 - Browsing changelog of a fixpack at Gibberlings 3 gives this amazing set of fixes.
Creature Sex Fixes - Various creatures were incorrectly identified as male, female, or neither.
Creature Gender Fixes - There exists a gender identifier for creatures in addition to sex. Gender has several different values beyond male and female, such as summoned or illusionary. Summoned, in particular, is what allows scripts to detect summoned creatures, the summoning cap to work, and for the death spell to work against just summons. Gender also controls the casting soundset of creatures.
I love this game.