First let's decide infractions. At least Player A gets a GRV for failing to instruct his opponent to lose 3 life (this assumes that he remembered the trigger for himself). The other will get a FTMG for failing to notice the opp's error.
There is no CPV, because CPV stems ONLY from violations of MTR 4.1 (as stated in the IPG 3.7 for CPV), and neither player misrepresented anything (failing to notice a life total discrepancy is not defined as a CPV). Player N did in fact lose life to his own fetch and so does not get a GRV for that.
Therefore, only a GRV for Player A remains. But the GRV is too old to back up completely, so we look to partial fixes as specified in the Additional Remedy for GRV, but there is no partial fix for life totals.
Therefore we remain where we are and we must rule in terms of (3 life) or (1 life). But Player A had an obligation to tell Player N to lose life - as a direct instruction on his Rhino effect, and he apparently failed in that obligation. Therefore, I'd probably rule (3 life) here.
Note that backing up through combat, adjusting life totals, and then allowing N to block again is NOT supported by the IPG. There is no partial backup at Competitive.
(at Reg I might entertain such a partial backup)
The main issue is that “not noticing a life total discrepancy” is not defined as an infraction, either a GRV or CPV, therefore no backup should occur for “failing to notice a life total discrepancy”.
This scenario appears to be a scenario where if both players are screwing up, lots of decisions are made using wrong life totals. There's going to be some rough justice later when we fix it. Lesson to players: verify life totals!!
(3 life) is my choice.
-Eric S.
edit: replaced square brackets with parens (square brackets are reserved, and essentially hide what's inside them.
Edited Scott Marshall (Jan. 4, 2015 09:29:05 PM)