You will get the Stronghold IF:
- You are a Paladin
- You go to the Windspear Hills, get blamed for killing Paladins, and rescue Garren's kid in exchange for his restoring your good name
- You return to the Order and speak with Wessalen or somebody
- You run back and forth across Amn doing quests for the Order
Garren himself has nothing to do with your actually joining the Order--he simply clears you of any wrongdoing--but the quest is required in order to make your name bubble to the top of the Prelate's Rolodex.
Killing Firkraag on your first visit has NO EFFECT on your obtaining the Paladin Stronghold. If you do, Garren is simply much more impressed than if you had simply saved his child. If you don't, the quest to do so is simply tacked on as the very last of your deeds required to enter the Order. You get a bit more EXP for not killing him until asked to do so, but that means walking around
without Carsomyr for a few more days.