1. Ooh that's a good one. I don't think I've ever had that happen. They would displace in order to form a legal stack.
2. If they came out of the first melee OK and still eligible to be a reserve, i.e. not in a ZoI and in good order, then I suppose they could be summoned to fight another melee. Stars and planets need to align though :-)
Are you just thinking these up or are these actually happening in your game :-)
It actually happened! . Terry, I'm just not that creative!
In my Lutzen game, I had a defending reserve unit that moved up to help a melee firefight stalemate result where the already disordered Prussian attackers then rolled a and then routed out of the melee. This left the defending reserve unit out of an EZOI.
On an adjacent Prussian assault against a stacked French battery and infantry unit (already disordered), the Prussians rolled another Firefight which then allowed the previous reserve unit to join the assault. The Prussians then rolled a and routed away again!
The stacked morale question was just one I thought of as I rolled for Stand and Close with my Assault stacks.