Here's a random thought I'll chuck out there FWIW. The outbound leg is 200 km plus right? What is the average distance between the waypoints? For example the entire leg could be 2 waypoints 200 kms apart, or it could be five waypoints at 50km intervals.
Would adding more waypoints help? Maybe the 'next' waypoint is to too far away for the AI to figure (with 'Escort' set at least)?
Just a guess...
And one more daft idea for luck!
Let's say something in the code precludes the 'escort' waypoints working after 200 kms, but the AI has plenty of fuel remaining. Could you re-design the mission so the fighters join up with the bombers at a later point, fly close-escort on the bomber stream for say 150 kms then break off to navigate under their own steam for the remainder of the mission - you could think of them as escorting for a while, before steaming off ahead on a sweep to clear the skies of the enemy?
Holy crap dude get out of my head!
I did just that. With the transports it was good enough to just add a spawn and then end point. With the escorts I had to make their spawn point (0) then set their Wpt #1 to the transports wpt #0. Then set the fighters last wpt to the transports last.
With that setup the fighters would leave their transports about half way through the escort and continue to their last wpt.
I added another wpt half way between the fighters first and last, for a total of three. Now the fighters leave the transports a little more than 3/4th of the way to the end.
Added another wpt and adjusted the previous one so that the fighters have 4 evenly spaced waypoints along the flight path, with only the first and last set to the transports. Went all 300+ km from Tunis to Sicily.
Seems they just needed some more direction...

I was really jazzed and came here to post it and then found theat hey...you came to the same conclusion!
By the way that cent med mod map is REALLY big!