A. Yes. This has been covered and happened before, late arrivals. You get a score of par + 4 for every hole you miss and you can finish the round.
B. Depends. If in a foursome then you might permitted to leave and return, receiving par + 4 for every hole you miss. If you're in a threesome however you won't have a group to come back to. Should you be forced to leave then one person goes to the card in front and the other to the card behind as twosomes are not allowed by rule.
C. No. If you're not there when the others finished then I don't see how you could complete the round. In this case it would be a DQ, either 999 or 888 depending on the circumstances for your departure. You can arrive late but you can't finish early so this would be a disqualification, no rating.