help! H4->F1->H4

My OPT will be expired next month.
1. Can I change my status back to H4? Is that too late?
2. I'm waiting the Canada immigration last approval letter, and plan go back to China after get it, or shold I go back before the OPT expired? (I'm afraid my H4 application will be rejected).
3. Is that easy to apply H4 after landing in Canada?

Thank you everyone!