It's a wonder the poor things haven't keeled over with all the chemicals you're surrounding them with lol. The only way I know to keep your cats "flea-free" is to keep them indoors totally and don't let them roam. That way they won't pick up fleas from other animals. Also.... I used to bath my cats on a regular basis, not they they enjoyed it (I had the scratches to prove it ha) but it did help to keep the flea population down.
Sorry I can't help further than that but you seem to have to chemical/herbal side covered and it's not working, so next step is to quarantine them for life!
cl60