Mark this is never a fun thing to go through but I think taking the time with proper treatment is the best way to save those fish and keep them for years to come.
I personally use uv with copper and have done so for years without running into problems. I like to use a slow powerhead for uv. Longer contact time means it would zap more crypto babies floating in the water column.
Its an interesting idea that perhaps frozen food may carry parasites. I don't think commercially available foods may pass protozoans like cryptocoryon. Although I have never studied this issue I do have a short story about the topic, if I may. Not trying to derail the topic here.
I used to run a hatchery outside the store for breeding fish and quarantining imported fish, freshwater stuff for most part. There I kept things so isolated that I had different syphons, nets, turkey basters... everything separate for each hospital tank. At times I would need to use something in another tank so I would toss the net or syphon tube into a large freezer. Thinking that if its frozen, stuff on it should be dead.
So one time my uncle came to visit us from another country. He is also a lifetime hobbyist and breeder who runs a fish shop. He sees me do this and before you know we
are debating on this. He claimed at that time that a cyst of ich can be on the net in bellow freezing temps for days, even dry he claimed they can survive for longer than we may think. To be honest, I never got around to finding out if my uncle was correct or not.