Personal foul?

[mbyron]

A live ball PF by B will be enforced from the end of the run if it was a running play or if the foul happened after A catches a forward pass. Either way, A gets to keep their gain.

The OP says that the B player "tried" to throw A to the ground. If he failed, I'd talk to him rather than flagging it. If he succeeded, I'd be more likely to flag it, depending on how severe the contact is.

Question 1: before or after the whistle is not the issue; you need to know whether the ball was live or dead. A dead-ball PF is enforced from the succeeding spot; a live ball foul might not be.

Question 2: already answered; contact foul cannot be USC.