Can I use a Master Seal to promote into a previous class' promoted class?

You do not need to change back into a Tactician to become Grand Master. If you use a Second Seal while promoted, you will gain access to all promotion classes your character is capable of.

Yep, you sure can. You can even Second Seal him straight back into his current class; he'll start again at level 1, with all stats and skills intact.

Combining the cross-classing along with unlimited missions, the focus no longer needs to be on ensuring you get every single level's worth of growth out of a character. Just get the skills you need from the classes, and then throw them into their final class. If, by the time they hit max level and the stats are still too low, just Second Seal him back into it, and keep at it.