it makes no sense to not give them a chance to recover.
they have an immune system too and even without meds some of them will recover, vet care just drives up the odds and helps them recover faster. When its caused by a virus, there is no medication that really works anyway.
i would give all of them a chance to recover, even without vet care some may pull through and fully recover.
i would consider euthanasia only for these that continue to deteriorate, when they get so bad that they have trouble breathing, or when they just fail to get rid of the RI after everything has been tried.
(and about the other topic: euthanasia means something like merciful death or painless death, and freezing is not that. humane methods i heared about would be an injection from a vet, and for a DIY method i often heared about pithing, which is basically swift brain damage. or you put them in deep sedation and then cause brain damage.)