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.
The problem here is that you're losing the in-direct fire support capability, and making a player choose between that and a more effective 360 area of denial unit/emplacement is... well, I won't say it but you can guess the word, homo, so to speak, and a no-brainer in let's say competitive playing, you'll want that in-direct fire support, but I think bringing in a mobile mortar team would help the game, competitive play and the more aggressive and mobile British players a hell of a lot more than hurt it and them, than just making the player decide between a static in-direct fire support emplacement and an area of denial emplacement.
Edit: No, the Bofors suppressive fire ability cannot compensate in-direct fire capabilities of a/the mortar (team) pit. Seeing all of this crying it's bound to get nerfed, it has far less of a range than the mortar pit, is again, static and cannot be used as frequently as the mortar (pit)'s barrage ability or auto-fire ability, auto-fire being it's ground attack so just barrage ability then.
Separating the Bofors and mortar pits into side upgrades and giving the Brits a mobile 3-inch mortar team seems to me like the best solution to the problem, the reason being, firstly, it's mobile, and secondly, it's mobile, the bofors and 17 pounder both have mobile counter-parts already available to the Brits by default, the Centaur being the more mobile version of the Bofors but not as effective against light vehicles and armor, or at least armor, not sure, haven't used it in a long time, because of obvious reasons, and the firefly, being the 17 pounder's more mobile counter-part, not to mention the Comet, being the generalist equal to the Firefly and a bit better armored while maintaining the cromwell's speed.
In the end, I think that the British players deserve to have a choice, between being mobile and going for a more static gameplay, if he goes more mobile, then it's like playing against any other mobile army, if he so chooses to go static, then fine, but he'll need to invest a lot of fuel, time, man power and map control for that, especially more fuel and time if the suggested changes here take effect of delaying the tech and increasing the fuel cost for the bofors and mortar pit.