Well, my idea is that the "disengage" feature can work as the assault resolution, select the unit and move to the hex that you want to retreat and then select the disengage button... or select the unit, push the button and then if all go fine you can see the retreat hexes.
Travel mode... well, i think that a retreat unit have 2 options, retreat to run to another place (evasion in travel model when you retreat) or retreat only one hex to a new defensive position (no travel model) both are posible but the first option means that the AT unit isnt ready to fight in the next turn and second it is ready to fight again.
Of course, if you want retreat an AT unit you need full movement points to disengage from enemy, cant shoot and retreat, or you save your AT unit or you dont save it (if retreated unit dont do it in Travel mode is a way to prevent a fighting retreat with AT).
Increase AT defensive value... well, in PzC terrain+seconday unit can stop a full division, add more defensive value to AT units may convert then in "bunkers" and AT units are defensive units BUT not the best units to defend against important attacks.
The point in retreat AT units is give then at least the option to survive when they are in contact whit a strong enemy, now AT unit are points sources for enemy because arent much efective against infantry (even armor can defeat then without important losses and if you use the "show the bunny" trick is worst... move first infantry to force AT unit expend shoot against then and move after the tanks) and need travel mode to retreat it means that you have an important chance to be Disrupted in the process and cant change formation in an AT unit D in enemy front means dead for the unit.
Well, leave the "old" boys talk about this, they have many wars in their backs