PROTIP: Always upgrade allied ships first. Once you have the defense to hold off the enemy, you can start trying to take out the mothership. Also using 3-way and getting close really helps against the defense orbs. But you need speed to keep them in your sights.
What is the point of Gravity Kill if you can only use it in story mode? Story mode resets when you get to Mars, so there's no reason to have infinite distance.
Killerinstinctss is right. This game is practically unplayable because it doesn't seem to realize I've let go of the keys. I already filed a bug report.