Hi, Humblefish isn't active on this forum any more. I can try and answer your questions, but our current quarantine protocol is here:
https://www.reef2reef.com/threads/current-quarantine-protocol.825055/
I generally do not recommend dosing praziquantel and copper at the same time. It can be done if absolutely needed, but it is usually best to dose copper and then prazi. In almost every instance, fish will die from protozoan diseases before flukes, so it is best to deal with them in that order.
Using prazi as a dip only works if you then move the fish to a new, uninfected tank. Dipping and returning them to the original tank just continues the infection. The problem with prazi is that it only kills the adult flukes, not the eggs or larva. This means it needs to be dosed multiple times in order to break the life cycle....2 doses, 10 days apart and sometimes even 3 doses. The other problem with prazi is that bacteria grow in aquariums that will eat prazi. Multiple doses over time in the same tank will make prazi less and less effective, One option to this is hyposalinity.
There is no evidence of copper resistant ich, but severe cases are more difficult to treat. Some people take too long to raise their copper level at the start, this allows the disease to get the upper hand.
As far as disease transfer on invertebrates, here is an article I wrote on that topic:
https://www.reef2reef.com/ams/biosecurity.812/
Jay Hemdal