Actually even without the mobile mortar, just separating the mortar and the Bofors into mutually exclusive unlocks is an awesome idea.
Which brings me to the following point: why would the AEC be behind an unlock anyway? I would say that the role and the effectiveness of the AEC do not require it being an exclusive in order to be balanced. In the beta its extra tech unlock may have been justified but the game has moved on a lot since then. Increase AEC cost by 50/10 if you must for timing purposes but make it always unlocked. It would have literally zero effect on gameplay, except a player could now have both the Bofors and the AEC at the same time, which I just don't see as some game breaking synergy. Yes a player could lock down a part of the map with a Bofors while harassing the other part with an AEC but the combined fuel cost of both means by the time this would be possible Axis should already have a lot of viable options to chase the AEC away
Bofors or Mortar pit should be the 2 mutually exclusive upgrades. This will solve a lot of problems, as already outlined above - without the synergy of both, they will be more manageable. You want the awesomeness that is Bofors, you pay by giving up indirect fire, and vice versa.
But I wouldn't mind any of the solutions outlined in this thread.
I don't see how giving no indirect fire if you go Bofors would be viable. Base Howitzers would need to be able to kill something then.