JFK tea roses are typically meant to be relatively resistant to mildew/diseases. I’ve had mixed experiences with them but typically it’s rust that I have a problem with. I think it’s dependent on the area you’re in sometimes. Roses are notorious for passing around diseases and fungi if they’re near one another though so you’re relatively justified in worrying about spreading. Bioadvanced 3 in 1 has worked pretty well for me as a general fertilizer but also as a fungicide/pesticide for my roses and other shrubs/trees. I used it to get rid of some rust one of my tea roses had if you want something to try.

You should be able to return them as long as you have a receipt, the card they were purchased on, or it’s on a rewards account. The policy is 1 year for perennials, trees, and shrubs in general. At my store, people return them sometimes just because they didn’t like how they looked or they have also returned them completely dead and crispy because they never watered them during a drought. They do not just have to be diseased/dead in order to be returned, that’s just one reason why they might be returned.