11spd Campag using an 11spd Shim cassette on a turbo?

It works fine. My summer bike is 11 spd Campag Potenza but when it is a mixed day I stick my winter wheels on from my winter bike. They are Shim and normally run with 10spd so I bought a cheap 2nd hand 105 11spd cassette, removed the 10 spd spacer and it all works brilliantly. So much so that when the Campag cassette on my nice Campag wheels wears out I will stick a Shim freehub on and a cheap Shim cassette. So much more VFM and there is no downside as most OCD freaks can't spot the cassette mismatch!

Yes you can run 11 speed Campag cassettes with 11 speed Shimano groups, and 11 speed Shimano cassettes with 11 speed Campag groups. I have both and swap wheelsets without even remembering which is which.

Yeah, plenty of people have done so, worth checking Velonews (and elsewhere) for various articles on this e.g. https://www.velonews.com/gear/drivetrain-compatibility-hidden-in-plain-s...

Technically there are fractional mm differences in cog spacing, but for all practical purposes it should Just Work. Also if this is for the trainer then any ERG mode stuff is only going to use a single front/rear cog combination anyway, so that'd be good with anything (preferably small chainring at the front)

I dont think so.  I used to run a campy cassette on a shimano hub using the 10sp version of these:

https://www.ebay.co.uk/itm/392447718199

However given the price you are probably better off just getting a Campy free hub for the turbo.  Elite, Tacx and Wahoo all offer Campy hubs for their turbo's.