Well, there is no manual-answer to questions 1 and 2. My guess is that
1. You are being pushed back, and suddenly the APU fails while engine #2 is already running.
Switch the generator online, once the engine has stabilized, so that the electrical buses are powered. Inform ground handling that they should stay connected and standby for further instructions, then inform ATC and do a PA-announcement.
Do NOT simply try to start the left engine with bleed-air from the right.
Then call maintenance and ask them for advice. What will probably happen, is that you tow the plane back to the ramp and shut down the engine after ground power has been connected.
Either offload the passengers, or, if the flight will depart anyway, you can still do a normal crossbleed-start without using the APU.
2. Shut down the starting engine immediately, to prevent it from being damaged. That's probably all there is for you to do. The plane won't go anywhere soon.
3. Engine failure at V1? Well, that's according to the Training Manual. Standard would be after reaching 1000', all recall-items completed, and aircraft properly trimmed.