OPT

1. Your H-4 application must reach USCIS before expiration of your OPT (although it could be argued that you have an extra 60 days, but I stay away from having to argue).
2. No different than filing a new H-1.
3. You could have a difficult time entering if cutting too close the end, although, legally you are entitled to it as long as you have a job in your field that you have been performing on your OPT.