I know how hard it is to lose a little one of any species. You're correct in that the bullfrogs are carriers and tads only have keratyn that it feeds on in the mouth parts for now. You're doing what you can but all we can do is provide you with our best advice. If you have to house them together then so it it. Boiled water removes too much of the needed oxygen from the water and I don't think it kills the fungus unless kept at a high temp for some time. Bleaching your old water is the the best thing to do to ensure it's not going back into the environment. Super care is what is necessary to keep your remaining toads chytrid free if possible.

If it ever does come down you having to let them go, I think it would be better to try to find new homes for them rather than release in the wild which you know is totally an irresponsible thing to do after they've been in captivity and possible carriers of the fungus. Worse case scenario is to contact your Wildlife Division of your State and ask them for guidance on disposal and they might be able to offer some assistance if you an't get anyone else to take them.

todley