when I see that whichever instance I choose is defederated from another bunch of random instances, is it possible to have those show in my feed without making a separate profile from other instances? and can I turn them off again if I see that defederating was the right idea?
You can just go to that instance’s webpage and view their frontpage. It’s worth noting that NSFW posts are often hidden without an account though, so if an instance was defederated for NSFW content it might not be obvious why.