1) I do not have any hard delay number for the 7842 IP module however worst case latency is at least one incoming line time.
2) There's is not real difference between the two approaches, you still have to store one incoming video line
3) Don't know, will ask the 7842 expert.
4) don't know, will ask the 7842 expert.
5) Yes, use the ADV8003/8005 over the ADV7842 ItoP converter.