Advice for downward, canter-trot transitions?

I agree, your horse is falling onto the forehand.

To do the transition from canter to the trot, I squeeze with my thighs, which is a general downward cue. My legs support active steps and balance. I don’t rely on my weight or really sitting deep in my seat, because those (to us) are cues for collection, and I want the horse to eventually collect with those cues. Sure, one should collect a bit before the downward to trot, but I don’t want my horse to trot off of my weight changes. I use my thighs to ask for the canter-trot and I stop following the canter with my body. Canter walk - I collect and think about a bird alighting on a branch from flight. Collect collect and then use my thighs to say walk. For me, this teaches a clear aid.

This is fine as long as you are interspersing this with asking her to collect and then asking her to extend. Otherwise you are teaching her that when you start to collect it means trot.

Well, yes - I thought that was implied. (Was trying to address the question on transitioning to trot).