You must maintain the same Heroes across the entire campaign of the app. If you have a varying number of players, you are going to need to share characters.
I think this is a missed opportunity by FFG.
Why? This is unnecessary. Let's say you play 4 heroes and then one player is not there, use the normal reward 3 heroes (legendary? I forget the name) and use the extra activation as the missing hero's activation. Problem solved.
This is not Descent, Imperial Assault does not scale the enemies to the number of players it scales player's health and activations.