I am sorry to hear this, like all you need is to deal with this issue. I get it though, I would have done everything you did as well. Fleas really freak me out, have had horrible infestations years ago, I finally moved I believe. As far as your neighbors, they might be very open to the suggestion of flea treatment/prevention. Clearly it is only common sense. I guess if you suggest it in an assertive, positive way it might go swimmingly.... or not. I'd give it a try at least, if they get rid of their dogs fleas it would be such a relief!! I hope they are nice, intelligent people for your sake, good luck