My two cents... if you are this close to plate that is missing a bolt head, I would fix it. It's going to be a lot easier to get access to that area now.
If your motor has decent compression, oil pressure and limited blow-by... there is no reason you could not get years out of this re-install. However, if it starts to leak later... trying to get rusted side plate off in situ is not going to be fun. IMO, those side plate bolt make that pan and head bolts look easy. I broke two off even with soaking, tapping and heat.
Well, at least there is one bolt that won't break off on you... so only 7 to go.