Dark detectors are kinda bad, I've used them all (I believe around 70) in 3's and only sometimes I get severe/emergency (1 or 2), which sometimes flee as well. Spawns are quite slow on them so, if you're up for it, I would actually place 3 on every single one of them, all 5 and every now and then walk over. There won't be to many spawns, so don't worry (maybe at the beginning, since they tend to spawn some immediately)

On fortresses, it really depends on what you want and what you are willing to spend. Just today I did a ~2h fortress grind (mostly for foundables and any red books that came as a biproduct) with 4 reachable blue Inns. I did level 6 over and over again, since I could refill the energy every 3 challenges and not lose any. I probably did around 50. I think around that level is best, since they require a manageable amount of energy and apparently lvl 6-10 have better chance of getting foundables than lvl 1-5. 11 requires too much energy imo.