Question of timing - Would this cause Snap to lose his regular action, as the additional white boost would then be red? Or can you select his regular action before the 1-turn red boost?

The former. He would do a red boost between his execute manoeuvre step and his perform action step. And thus be stressed before the perform action step.

Thought so, wishful thinking I guess! Thanks!

For what it's worth, Snap would still get a perform action step , he just can't perform most actions in it because he will, as @thespaceinvader notes, be stressed (since "after you execute a manoeuvre" comes before your perform action step).

Primed Thrusters would allow him to Barrel Roll, though - and following up a manoeuvre with both a 90' boost and a Roll could make Snap impressively squirrely.