yes, excess bathing will probably reduce efficacy, but supposedly if your dog has a healthy layer on natural oil when you apply the product, and then you wait a few days, it should be water-proof. but it’s not working anyway (frontline) and i wouldn’t leave fleas on my dog to bite and hopefully ingest poison and hopefully die. i would just bath the dog with something not super harsh.
since your dog has some special issues, perhaps you should consult with your vet. i can schedule appointments with techs at my clinic for free, and they aren’t dummies. they will also consult with me over the phone for free. the bummer with confortis is if they puke, they are puking up the pill and it’s pricey. but i’ve not had that happen at all. i don’t know about more severe side effects with that one, but you definitely have to weigh the pros and cons of treating your dog with an insecticide and asking him to live with blood-sucking parasites. if your dog has allergies, health issues, etc, i’d check with an experienced pro, but regardless, you are taking a risk either way - whether you treat or not.