will not re-ignite, but that's OK. So long as you get all three fires. (you do not have to go back to the ones you arleady lit).To get grognard and his army to appear, you must "leave the area", To do this AFTER you have lit all three fires at least once, go into Castle Harmondale and then come back out. Since the Castle is considered a seperate "area", you did, in fact leave Harmondale proper and returned.
The Goblin group you are looking for is nowehere near that fort and the fort chest had nothing to do with them. When you find them and kill Grognard, he will drop his sword (which, incidently, is not a bad weapon for a low-level party) and you can now use the stables.
I won't spoil you on where to look for Grognard, but I will tell you he is in the Harmodale counryside somewhere.
