I went back to play a few turns of TWIN - I could not remember playing it under 3.4. And I found something that may or may not have impacted the ETE :
Playing as Allies vs PO, on turn 1, I did my usual routine of maneuver with the most powerful units to RBC as much as I can, flank stubborn defenders and launch attacks with those powerful units not yet moved. Good plan. Great fun playing RBC checkers. Too bad turn 1 ended after 1 round, with only 30% used - (dang those airborne for continuing - I said LL) !
I had forgotten that the support units (naval, air) participate to a great extent in the overruns, and in responding to enemy moves in reaction to overruns. So many of my air units were down to 60/60 readiness/supply, and the navy was in worse shape. When the supporting units lose a lot of quality during the turn, it increases the chance of ETE - just like attacking with tired and/or weak ground units.
I tried it again, this time setting half of the air to Rest, and moving half (every other ship) of the navy to just out of range of the coast. Did all my maneuvring again, not as successful. Having fewer battleships makes a difference.
Then I shifted the navy so that those near the coast moved out; those resting, moved in to support. Checked the air, and any active air unit under 90/90, I set to rest. Did my attacks - using LL for consistency. Again the airborne gets enthusiastic, so 30% of the turn is used. But - No ETE. And this with the default MRPB.
So, in my case, I found the MRPB was not the sole source of the problem.
Looks like you have a solution, and that's great - just some more data for the possible use.
Oh yeah, if you're editing this scenario - you may want to add a supply point for the 9th AF -- they did not need one under the old supply rules, but they do need one under 3.4. Of course, you could always play under old supply rules, as an alternative.