1. After years of intense dislike of each other, maybe jumping into team tactics would be a little difficult.
2. Considering the distance and computer processing power involved, it is doubtfull that Sharron was controling the Ghost actively. Perhaps she copied herself or a mission into the Ghost's AI.
3. The YF-19 was in robot mode when it crashed into the top of the SDF-1. It went in with it's shield covering the head in a dive. Considering the robot form is much stronger than the aircraft form, plus the pinpoint barrier system, I found this event plausable.