scrimmage kick by K 1 is very high, so R2 signals for a fair catch, he lets the kick hit the ground. After the kick hits the ground he decides to run. What would call?

Canadian Ruling
 

Since we don't have the fair catch rule, I've got nothing, unless there was an offside member of the K team within the restraining zone when the R player touched the ball.

Off the top of my head it is a delay of game. Once a member of R gives a FC signal, vaild or invalid, and a member of R recovers or gains possession, it is a dead ball. Running with the ball is a delay foul.

Did you mean, "he decides to run with the ball after recovering it"?

The ball becomes dead (but is not a fair catch) as soon as R recovers, after any R player gives a fair catch signal. Technically, a player who runs with the ball in this situation is guilty of delay of game, but it's another one of those situations where I'd be hard-pressed to call it.

I would blow the ball dead as soon as R secures possession of the kick. The play is dead at that point and there would be no need to call a foul unless R delays the game by continuing to run. If he stops at the whistle, there is no foul.

I use the same tolerance that I would use if R didn't signal for a fair catch and caught the ball with one knee on the ground. Blow it dead and no foul.

Exactly what JDLJ says... Get on the whistle to kill the play ASAP. FC signal means No Return no matter how the receiver fields the ball.
If he takes off and runs then gets whacked, too bad.

Delay Foul only if he does not stop within a reasonable number of steps after the whistle.