The Dominion is an organization that effectively introduced itself by committing multiple acts of mass murder, and disciplined its "subjects" by infecting them with genetically engineered atrocities. The Federation should not try for reciprocity with it.
Agreed. The Federation should live up to its own values and treat Jem'Hadar prisoners humanely.
The closest thing the Federation could to to allowing Jem'Hadar captives an honorable death is to simply lock them up together and let them snap each other's necks.
Even that is too far. If a captured Jem'Hadar is a danger to himself or others, then he should be restrained from harming himself or others. And if the Federation cannot obtain a supply of Ketracel White, then it should put that captured Jem'Hadar into stasis until he can be repatriated to the Dominion after the war (or given the opportunity to request asylum in the Federation if there's reason to believe he'll be executed after repatriation).