For the Enterprise A, I had just gone with the thought that a different Constitution-class vessel, maybe Yorktown, maybe Ti-Ho, had been renamed Enterprise. It would have been a newer ship, launched perhaps close to the V'Ger Incident, and possibly was in Spacedock for a refit to a newer bridge and computer system. That would explain the new touchscreen bridge that looks like the older Enterprise's Bridge. This ship would've been disabled during the Probe incident in Star Trek 4, and available for re-assignment as the new Enterprise for Kirk and crew. The ship hadn't undergone a shakedown with the new systems, explaining Scotty's trouble with the ship. The second bridge would have been available from some other Constitution-class vessel or at Spacedock, and put on the ship in hopes that this newer bridge would have sufficiently advanced control systems for the ship, though without a proper shakedown, the Enterprise would still experience the issues it found in Star Trek 5. By Star Trek 6, they had had one additional bridge, and the ship was running well, even though it was about 20 years old or so, leading to being decommissioned in favor of the Enterprise B. That worked for me.