I wouldn't cut/have the barrel cut. The gas system is set up for a rifle-length gas system and you would also more than likely have to alter the gas port size to keep the rifle functioning properly.
Have you already installed the MI handguard? You want to assure that you have the proper barrel nut for the LAR-8 and its related Bushmaster models.
Yes, you could transfer the forward assist and port door, but they are included with the upper receiver assembly as well. The plug is primarily for use when using iron sights. we use a longer elevation index to achieve the full range of sight adjustment. The plug screws out of the top of the upper receiver to allow that longer index to actually go into the receiver where, without the removable plug, it would bottom out against the top of the receiver. I recommend having it in place to keep debris out of your receiver/carrier path (even with a scope mount in place, it will migrate into the receiver eventually).
Steve/RRA