#1
So for a while my JCM 2000 has had no problems, but I accidently plugged the output of the preamp into the input of the amp and I couldn't figure out why my guitar wasn't working (for those wondering, I wired up my pedals wrong and that's how it happen) and so 15ish minutes later i figured it out and plugged it back in normally. I didn't notice any difference but could this of done any damage to the preamp section?

And yes...I feel dumb for doing that
#2
you are fine. it just instrument or line level.

now if it was a speaker cable to the input, you would be having some problems.