[Qthulhu]

From my reading, in both situations it's hard luck for the infantry as the movement rules would take priority over the overrun rules.

You could justify basic self-preservation as an in-universe explanation for why your men could dismount despite mounting in the same turn, but you would still come back it being a way to fudge the rules to allow your troops to almost fly around the map.

If we assume that the in-universe reason they cannot dismount in the turn they mount is because they're securing themselves on the vehicle, then the in-universe explanation for why they cannot bail is because they've only just secured themselves when they notice the driver is ramming for the greater good. Too late to bail. SPLAT.