You can't replay any mission in showdown, what happens is what happens. You get 15 outs on the final showdown I believe. For any RED boss you are only scoring runs so read who the pitcher is righty/lefty and then stack your lineup accordingly. You will not play the field at all so you could have Trout as a Catcher and that's fine. For those final showdowns they only use 1 pitcher so you can take till you get 2 strikes if you want to lower the pitchers stamina which will help increase your PCI size.
Generally I'll get 1 silver bullpen guy to help with the pitching missions and pretty much all drafts go for lineup upgrades.