If I purchase AMC tickets from Costco, are you able to reserve seats or do you just show up and hope there are seats available?

As long as you have an online copy of your ticket it shouldn't be a problem

Yeah, you have to show up.

I believe with the paper tickets or outside tickets you have to sgow up and redeem, rather than reserve online.

Those the tickets say you can used it for dine in ... I truly doubt it