It almost feels like I'm in a different fight as we always have the Trenchcutters aggro on the healers after a few seconds of spawn tank aggro (even through after guarding them just to make the switch slower) and never the Warriors. Obviously the healer just explodes if the trenchies aren't AoE taunted in time and we are pretty much ****ed on the last group.
Another thing I have noticed is that the walkers skips a phase of saturation fire when downed. I.e. If it was shooting right (his right so away from the entrance) then goes down he will shoot mid after he goes back up rather than diagonally between right and mid, anybody else seen this?
Whats happening here is the walker is just barely beginning to start shooting cone 4 (the middle right one, as you call it) as he is getting knocked down. If he starts casting this at all it means he will use cone 3 (the middle one) next after he gets up. Beating the calibration droid phase slightly faster should make it so he does NOT start cone 4 before he gets downed, and will make him use cone 4 after he gets back up. You just need to squeeze out a bit more damage on the droid phase.
As for the Trenchcutters I don't know what to tell you. They always run right for me and ignore the healers, and we don't have the healers guarded. Back before we switched to this strat we tried to have DPS single target the warriors down, then kill the Trenchcutters. Doing this caused me to lose threat on the trenchcutters because it took too long for me to be able to attack them. Using the behind the walker, tank guarding other tank strat we don't have this problem though.