1) Plenty; worst case I've had recently (1 hour wait at immigration) I still had time for a shower in the domestic lounge before a flight with a scheduled 2 hour connection

2) Not sure but probably, given they are both *A. However, you will still have to collect bags and clear customs at JNB, then recheck, so it's rather moot.

Lots of info on connecting at JNB in the Africa forum