I sympathise as I bloody hate fleas, but I do feel for the poor cats! I'm reading this thread with interest (you'll see why below!). When we first moved into this house, we had a flea infestation (possibly because previous owners had dogs/cats, and then when I put heating on, it kick started everything). It took me weeks to get it sorted. Nightmare when you're unpacking, working FT and have two kids (we didn't have cats/any furry pets then!). Diatomaceous earth (the food grade stuff), flea bombing the place, vacuuming daily helped then!
I now have 4 cats (a Maine Coon, two BSH and a part MC/part god-knows what). For the first time, I found fleas on them in early September (despite being meticulous with applying the vet-prescribed spot on stuff for them every month, daily vacuuming and so on). The only thing I can think is that we'd gone on holiday for 10 days in the fortnight preceding this - ergo no one home to vacuum place - and also the vet had changed the spot-on prescription from Prinovox to Stronghold Plus for two of my cats (of the other two, one is still on Prinovox for the moment, and the MC is on Advantage). Maybe that made a difference to things.
Anyway, I freaked out and phoned the vet about it. Vet told me that loads of people had phoned in about flea issues that weekend. From what I understood re: spot-on stuff that the vet prescribed - the flea has to jump on cat and then feed on cat, and only then is killed. Soooo, whole place was sprayed with indorex, flea bombed, everything washed, cats were combed through and given the flea tablets in addition to their spot on (cat eats tablet and then fleas will literally drop off the cat and die). Problem went away.
I found a couple of fleas on the MC this weekend, during my weekly comb through/check (I have a metal Nitty Gritty comb for the kids and check them weekly as I'm paranoid about nits: since last month, I also have a separate metal Nitty Gritty comb for the cats - they're weirdos and actually like being checked/combed through with the Nitty Gritty!!). So extra load of indorex sprayed about. Flea traps have been put in the different rooms. I've ordered some food grade diatomaceous earth to sprinkle under/between floorboards in my house (all original wood floors).
One thing I've not thought of is that fleas live in the garden. My cats have the freedom to go in/out the house during daylight hours, so it could be that the critters are hitch-hiking a lift in from the garden via my cats. I read that nematodes can kill fleas and this time of year is a good time to start treating the garden (putting nematodes in to garden), so that will be my next port of call. As an aside, if anyone has tried this, I'd love to know if things improved!
Weirdly, a lady I know has two cats. She doesn't give them the spot on stuff from the vet....she only gives them the flea tablet (that makes the fleas fall off the cats!). And she's never had a problem with fleas.
TLDR, many sympathies to you OP, and of course to the poor cats. I really hope that you can get things sorted for the sake of the cats as well as yourself. I've tried a shit load of different things in past few weeks. Shout out if anyone else has additional ideas.