yes

No. You can't claim that treaty which targets the group of students who received their F1 visa originated from China. If you changed your status from F2 to F1, not having a F1 visa, you cannot claim that treaty. If you changed your status from F2 to F1, and got a F1 visa from a third country, such as Mexico, or Canada, you still cannot claim the treaty. Only if you went back to China and got a F1 visa, you can claim that treaty.

nope, you can not, since you changed your status here, you can not claim it.

How about from H4 to F1? Still cannot claim the treaty benefit as from F2 to F1?

No. See my post above. You must have F1 visa issued in China to get this treat work on you. Otherwise, no way. You can claim that treaty and you can take the chance that you will not be caught by IRS. But once you are audit by IRS, you are done here.