MTG Salvation

I strongly discourage people from using dice with such creatures, regardless of the kind of deck. Not only can they be confused with counters, they cause communication issues. You should be able to look at both graveyards, which must remain clear and visible with no cards piled and hidden, and keep track that way.

The kind of information you're asking about is called derived information. At regular REL (FNM and the like), this is considered the same as free information like life totals, cards in hand, etc.: things that cannot be concealed and that both players must have full transparent access to at any time. Dice are somewhat less of a problem there, but then they should be updated by both players constantly, a noticed discrepancy should be corrected immediately. At competitive and professional REL, derived information still cannot be concealed or misrepresented, but the difference is that your opponent is not obligated to help you calculate it or figure it out, if you ask, he's allowed to say "well, see my graveyard...".

The problem with such dice is that they are a representation, a constant statement. From a communication standpoint, it can get pretty blurry when it comes to determine who is making that statement, the player with the creature, or the opponent because it's his dice that he got out as a kind of note and manipulates to keep track of the opponent's creature's size, or both players. If it can be construed that you were responsible or co-responsible for the dice's state and the statement is at least partially made by you, you cannot intentionally leave it at a wrong number and try to "get" your opponent that way, making him do bad attacks and blocks based on that (if it's clear the dice were entirely his thing and he didn't ask you anything, you can say nothing and benefit from his mistake, but it's often not that clear-cut). Intentional misrepresentation of free or derived information is cheating. If the judge genuinely believes it was not intentional and really an oversight by both players, game actions such as bad attacks and blocks made based on that false info can be taken back.

As for question 2), if both genuinely thought the Goyf should die at the time, it's a game rule violation for the Goyf's controller who should have put his creature in his graveyard, and failure to maintain the game state for the player who cast Dismember (those result in Warnings at competitive and professional REL only, there are no formal warnings at regular REL). If the Dismember player knew this shouldn't have killed the Goyf, this is cheating, of course, and automatic disqualification if a judge believes it to be the case. Absent that, as for repairing the game, the judge will determine, in neutral collaboration with the players, what would be the best thing to do, it really depends on the exact situation and how much has happened since. A backup might be possible if not too many things have happened since the event, up to not casting Dismember possibly if that move was made based on information that the Goyf's controller was supposed to provide correctly and didn't, but this one aspect can be blurry and arguable, it should be determined with all aspects of the situation taken into account. But going back one full turn with decisions taken, cards drawn, etc., is sometimes not possible. In most cases, the Goyf should at least be put back on the battlefield and be considered as it never left it, unless it makes no sense because of important factors. There's rarely a perfect solution to these problems though, the judge and the two players must assess the situation and the judge must find the best solution he or she can come up with within the parameters of the tournament policy documents.

If you're interested in reading about such things directly, these said policy documents (Tournament rules, Infraction procedure guide, Judging at regular REL) are here.