I am playing a Fighter/Mage multi class and my fighter class was inactive when I finished D'Arnise the first time so I was not offered it as a stronghold. After doing a lot of other chapter 2 quests I decided that was lame but didn't want to go back to my old save. I was able to get the stronghold by resetting the quest.
(This uses cheats - I modified the save file using EE Keeper running under Wineskin on OSX, should be easier on Win).
By investigating my old saves, I was able to determine various global variables:
ARRIVEDNALIAKEEP = created when you accept the quest, value 533897. After you arrive at D'Arnise, the value changes to 910166. After you enter the keep through the secret entrance, value is 1105359. Then it keeps this value for the rest of the game.
NALIAINLIBRARY = created some time when you're exploring the keep. My value was 2. I think it is related to her family servant who you have a chance to save.
NALIAKEEPPLOT = created once you turn in the quest, value 1.
NALIAMESSENGER = created once you turn in the quest. Not sure what it does, but the value is 2069566.
SPRITE_IS_DEADNALIA = 0, created some time in the middle of D'Arnise
SPRITE_IS_DEADTORGAL = 1, created when you kill Torgal
There is also several SPRITE_IS_DEAD counters for troll types. Not sure what they are for, they count up as you kill trolls in the keep.
Steps to gain Stronghold, after you have turned in the quest:
1. Backup your save file!
2. Edit save file: delete the variables ARRIVEDNALIAKEEP, NALIAINLIBRARY, NALIAKEEPPLOT, and NALIAMESSENGER, and SPRITE_IS_DEADTORGAL. I think that only a subset of these are necessary to delete, but I didn't test enough to find out which ones matter.
3. Find Nalia in the Copper Coronet.
4. Kill Nalia using ctrl+Y
5. C:CreateCreature("Nalia8"). This new Nalia will offer you the quest (the old Nalia would never offer you the quest). You could probably create her at any level, if you wanted her higher level to play as a character (I think 11, 13, 15, are available).
6. Accept quest from new Nalia (I declined to have her in my party). I did not test whether the keep is reset for re-looting when you do this, but it does say that the world map is updated so you might be able to return.
7. Recreate globals SPRITE_IS_DEADTORGAL=1, ARRIVEDNALIAKEEP=1105359, NALIAINLIBRARY=2.
8. Talk to Nalia, and she will accept you to turn in the quest, netting you 45k XP and the dialogue where she offers you the stronghold if you are the fighter class. You will be teleported to the lower level hall where you will talk to the steward and have to travel back to Athkatla. There will be no loot in any chest, just a few people to talk to for flavor. This will automatically recreate the remaining globals that were deleted - your game will be the same as it was except you will have D'Arnise keep and a mysterious Nalia corpse that everyone ignores!
My game had a bug where I couldn't keep the front gate open. You can exit the front gate by having someone stand below when you open it, then it stays open until you move off. Or you can always use the secret entrance to get back in.
Final note: it didn't work for me to just set NALIAKEEPPLOT=0 using the CLUAConsole and create Nalia8. She wouldn't offer me the quest.