I'm finally addressing gashlycrumb's big question with the pictures (sorry for the delay). Too many answers/opinions, etc. follow it, so I'm not reading them (and not discounting or minimizing the vigor of the debate and desire for answers) and just going straight to my analysis w/o looking at them, as it seems to have turned into a snowball of speculation. I hope to provide a straight answer and end the debate.
1) It can occur at either time. The mounting would be a 2nd action, costing 2 MPs, thus this action opens the MMC up to two OFs. So it's up to the NVA player.
2) Not sure I fully understand the question. The Sequence runs through for the entry of the MMC into the hex, then it begins again for the embarkation. Does that answer your question?
3) It's the same as just going through the Sequence again but you know there's no events, mines, ffe, etc. Your added step is fine, though.
4) Incorrect on the M48; it expends half its MF (thus 6 MPs) to allow the embarkation. The action of the MMC activates the M48 (which I'm assuming is Open by default here) and by the example is put under AM. The MMC is subject to 2 OF attacks, the M48 to 6 (despite the absurdity and inconceivability of this, not to mention low-percentage tactics and wastefulness of available FP). All OF occurs here. The NVA would be silly to attempt an attack on the M48 as its now technically Moving and its lowest armor is 3, so really not a wise play. Much better to fire at the MMC.
5) Why would you mount a tank only to have it fire right away and force the MMC to dismount. Tactically unsound. The answer is don't do this.
6) Though I'll admit this is not clearly stated (and even though it's a consequential and not an impulse/action move) it's the equivalent of the MPs they would have to expend in the hex, e.g., 1 in Clear, 2 in Light Woods. The OFs, btw, are attacks not units, so it can be one attack by three units, just to be clear.