Transmission reinforcement plate

It is an important part as it does a few things, it acts as a shim to space the converter away from the front pump in the transmission to prevent damage to the pump, it acts as a starter shim to help position the pinion on the ring gear as well. It's last purpose is to cover the opening in the bell-housing between the engine and transmission to keep dirt and junk out of the bell housing.

You're welcome. The only one I really would worry about is damaging the transmission if the converter is in the front of the pump too deep as even an eighth of an inch can cause issues. As for pulling it all apart, you could cut the tops out of the bolt bosses, loosen the bolts and slip it into place after you pull the starter out. That would also allow it to be removed easier if needed. Just be sure that all of the plate is used, because of the way it's made if you only installed the lower section it could create an angle between the transmission and block that would but stress on the flexplate, input shaft and internals.