The long story is spoilered below, but in short, I tried reclaiming with a sevently-strong army, who killed both the demons remaining in the fortress, but because they were all inexperienced soldiers, and all friends, the few losses the suffered sent them into an unrecoverable tantrum/madness spiral, and ended up having to abandon the fortress again.
I had a very nice, well-functioning fortress, until my miners delved too deep and awoke the nameless terror lurking in the depths.
I almost managed to beat them off, thanks to some particularly heroic actions by my elite military*, but alas, with only one demon left, all my best troops were either dead, or too injured/exhausted to keep going, and the demon was free to stalk the corridors, killing at will. Eventually, over half my dwarves were dead, almost all the others were hospitalized, several were wandering around babbling or berserk and locked in their rooms (and my tax collector was still wandering around trying to collect taxes).
*Particularly mention needs to be made of one elite soldier (who on a previous occasion had single-handedly slaughtered two elven ambush parties and driven off another when he got locked out of the fort during a multiple ambush) wielding an artifact shortsword, who killed two demons (one single-handedly) and wounded several others. Also special mention goes to his lover, who managed to tank 5 demons simultaneously. Amazingly, both survived these encounteres, but met an ignominious end not long after, dying of thirst in the depths on the mine, as they were too injured to move and there were no mobile and sane dwarves left to give them water.
Eventually, with all my dwarfs dead or dying, I decided to abandon the fortress.
Having first visited it in adventure mode (dying to a giant rat which bit my adventurer in the heart), I decided to reclaim the fort. The first two reclaimation parties (each of only 7 dwarfs each) both got slaughtered. (The first lost a dwarf to the same giant rat - now elite - that killed my adventurer, and were then wiped out by a fire demon that I hadn't seen before. The second party was wiped out by the tentacle demon that destoyed my fortress originally).
My third reclaimation party (70 dwarfs) managed to kill both demons, losing 5 dwarfs in the process...
And then collapsed into a mess of tantrums (some starting before the second demon was dead) and madness (9 dwarfs subsequently went berserk and killed several others, several more went melancholy/stark raving mad), and I ended up abandoning it again.
Which is honestly pathetic, because drafting my entire original civilian workforce to combat the demons, and losing half of them, caused less unhappiness.
I've started another reclaimation attempt, this time with just 7 dwarfs again, but this time I immediately walled off the entrance to the old fort to keep any nasties inside, and am going to build a new small, separate fortress, until I have got a physically and mentally tough enough military to risk entering the depths.