This sounds like a very good strategy, I'm not sure how some others aren't getting it. Having completed this trial on cannons, regular DPS, DPS/ harpoon, and tank this all sounds good. The last time I completed it (the normal way) we actually intentionally used the WHM to pull to the tank since mobs tend to flock toward the primary healer immediately anyway (except the Biasts that can mostly be ignored as you said). I'd considered something like this but never got around to working it out/ wasn't sure it would really work. I can confirm that the larger adds definitely will stray as far as the beginning of the trial if they have a reason to, so it only makes sense that you could hold them there completely out of the way. I was also a bit stunned to see that the regular adds do indeed despawn if left alone long enough (apparently when the next wall breaks).

I'd think the only wild card would be the last large add (forget his name, but final boss for SV HM) who doesn't seem to have any sort of enmity table and spawns near the end. Had him take out a cannon once because he was just so dead set on attacking that person for no particular reason.