Breakthrough Balancing + Mortars

You are mistaken about there being no counterplay to the mortars.

First of all there is the recon drone that can fly past the enemy defenses and delete the mortars, repeatedly. They just patched it so that the anti grenade gadgets are not able to shield the mortar from the drone.

As a stopgap measure there is that gadget that can block at least one hit of the mortar.

If you can squeeze into the main sector, you can also disrupt them with your own mortar.

Then there is the classic counterplay of not standing where the mortar is aiming at and to Break-through (get it?) the defenders lines or to flank around them. The damage a mortar does is low enough that it takes 2 to 3 close hits to go down from 100% which means that someone who does not want under any circumstances to be taken out by a mortar, has a lot of time to get away.
I do not assume that  there are like 5 people using mortar at the same time on the same place because that is a unicorn scenario and even then these people would depend on the rest of the defenders to keep the attackers in place and not letting them get out of the clusters that the mortar users love to aim at.

Other than that in the case of blackwell, I think they should pull back the attacker spawn  boundaries a bit in order to make it a bit easier to counter mortar. Maybe they could make the attacker boundaries more Y- shaped in order to have room for a mortar in the middle as well as more protected areas in the left and right extremities in order to make it easier for the attackers to flank the defenders.