I don't know you personally. I don't know what else you had going on today. What I do know is that skipping that mechanic and sending it off is the smartest thing you did all day.
Skip tearing it back down after an hour (or thereabouts). You are paying (whoever you are paying) to get it right. Are you buying the piston from the replater? Usually, one DOES do a fair bit of cleaning before reassembly. Read up on the white cloth and oil checks after several soft scrubs with Dawn (or other).