When you are transporting a ground unit here's what I understand
- active the ground unit and pay PP
- use a convoy and if stacked also a surface fleet for escort
- leaving the port and entering a port don't cost a movement point
- movement points are 10 counting for the convoy
- once movement is done with the convoy a sortie is added to the convoy and surface fleet if one escorted. Any remaining movement can be used for the convoy/escort. So the convoy/escort can only make it back to the port of origin (embarking unit) if it has enough movement points?
Correct. The convoy/escort could also choose to move to a different port provided it has the movement points to reach it.
Or activate using a sortie to make it back as another option?
If the naval units wanted to go back to the port of origin and didn't have the MP to do it with the initial activation, then yes. This is how they could make it back.
So my other question is can you use the convoy again (another sortie) to move the ground unit being transported further?
The convoy/escort could activate again to move a different ground unit or an air unit. It could not activate again to move the same ground unit. That's because a ground unit can only activate once per turn to move.
I'll have to check, but I thought in a tutorial video on a med scenario, there was an example of an air unit acquiring more sorties to be moved another 10 movements to make it to a Egyptian port.
An air unit is different in that as long as it has less that 6 sorties, it can activate multiple times to move. So in your example, you could activate the air unit, followed by convoy/escort multiple times to get to a destination. Each activation costing a sortie (assuming no combat).
My guess is the air unit didn't need a convoy and just activated two times to move up to 20 movement points (across seas zones and sea boxes)?
An air unit cannot move on its own between Map Boxes. It requires a convoy to do that.