In fighter mode, there were not enough improvements to make a difference...it still will not lock in place between the arm/leg area......in all fairness, it has always been pretty solid in Gerwalk and Battroid modes....and that continues with the latest versions....
As for the SAP arm armor's gun bay door.....there was a second set of arm armor included...it was packed separately in a small box...did yours not come with them? The second set has a redesigned locking tab that will keep it locked when closed.....
ET should have come up with a way to use magnets to keep the arms attached to the legs when in fighter mode....missed opportunity to have implemented their magnet expertise I suppose....