Were you in full-fullscreen? As in, did you click the uncolored expand icon and then the blue icon? If so, and you still can't see the UI, that must mean an undiscovered resolution issue. In which case, what's your screen resolution?
As for the boss, the most dangerous attack is the green homing bullets. Feel free to sacrifice time to avoid them. As long as you kill some of the chasers and pick up their oxygen, you'll have plenty of time.