Two Questions

[dumbref]

Play 1

This is tricky, but there is order to the process.

2-30-15 states: ... a substitute becomes a player when he enters the field and communicates with a teammate or an official, enters the huddle, ...

2-30-12 states: A replaced player is one who has been notified by a substitute that he is to leave the field. >>>

3-7-2 states: ... Replaced players shall leave the field immediately.

The key words that ties this together is communicates and notified. "leave the field immediately" does not begin until the communication happens. We should allow some reasonable amount of time for that communication, yes even confusion to occur. We have drawn the line in the sand at breaking the huddle with 12.

Play 2:
I can not find a rule against clumsy. If the contact was unintentional and did not affect the play, it is incidental contact in my book. Now, the B player does not know that. So getting to this situation to insure it does not escalate into something much worse is paramount. I’m not sure anyone has made that point.

Another point is see the whole play. Know what cause the trip.