"cap around him" this doesn't work on 90% of maps. Mortar/Bofors barrage usually still in range.
"Dont let him get to entrenched easily". Once the first bofors is up there is nothing you can do to stop it multiplying since you wont have the tools yet.
"Brummbar or Jp4 will chew up emplacements very quickly." I don't know what level you play at, but I'd advise you to play a top 50 brit player who uses this strat and you will quickly find out why they don't work.
Sure it does. Mortar pit won't stop you from capping , stand on the edge of the cap zone and he has zero vision of you. Once you neutralize the territory it's irrelevant. Bofors barrage has a relatively small Aoe and you can avoid it if you are paying attention due to the long time it takes for it to hit.
Getting the first bofors up requires him to win atleast two engagements. Once he DOES get it up you do need to pull back and play defensively. Keep in mind that the bofors can only cover a relatively small area with its attack and a medium one with the barrage. (Which requires a unit inside the bofors or for him to spend 250 manpower on a forward assembly.) Essentially, that's two squads worth of manpower committed defensively. You can leverage that ~500 manpower differential to overwhelm him elsewhere on the map.
I mostly play players in the top 100-200, although admittedly I haven't played a ton of games against this sort of strategy.