I absolutely agree that there's an underlying training problem here; and I am not a person that believes that tack of any kind is the total solution. However, in certain, limited circumstances, tack along with behavior modification/training can be part of the solution.
Normally, if I had a horse that lugged, leaned and braced on the bit; I'd be doing a lot of flatwork with a lot of transitions; putting the horse on a circle and pushing off my inside leg onto the outside rein and "springing" the horse off the inside rein. I'd also include lots of hill work to get them working correctly off their hind end, and maybe include some ground driving or lunging in a surcingle and cavesson.
But the OP has stated the horse is coming back into work after an injury, she doesn't have a ring to ride in, the horse is up and excited, and she's trying to hack down the road, and the leaning and bracing is creatiing an unsafe situation.
That's why, in this limited situation, I think a bit that discourages the behavior is *part* of the solution. Because the sound, solid, tried and true classical solution isn't an option for this rider at this time.
The OP needs a way to hack her horse down the road while legging back up that's safe for both of them and hopefully less frustrating. The triangle snaffle may work or it may not. And as I've stated previously, it's not a bit you should ride in consistently. I'm just saying it may be worth a try to get her through this difficult period.