Go under My Pass on Rail Planner, the validity period is what they may want to see. I've taken ferries multiple times and never had to prove I had Interrail/Eurail (other than my booking).

No need to activate a pass day in order to get a discount btw.

I can't really help for your other issues, sorry.

Usually they don’t even ask for your discount (with a few exceptions), otherwise do as thibcabe suggested