Can inconsistencies with the left to right (horizontal) angle of the horn cause inconsistencies like cracked notes?

That is looking from the side the horn would be moving in a predictable manner, up and down or whatever the player does vertically, for range, but from the front the left hand would be traversing the horizontal plain and maybe not so predictably.

Yup, that's part of Doug's pivot system- it's not just up and down, it's all directions.

I’m hoping, after decades of trying not to think about things too much, that I can make some late in the day gains similar to those I’ve made in running and weight lifting since starting a few years back. Understanding and practicing good form in athletics (and doing indirect work to aid form development) has led to becoming healthier in and better at those sports. A lot of the form has also become automatic or very easy to feel as “in the slot” or not.

Hopefully it’s not too late to become mindfully-automatic in more aspects of trombone playing.

That's what I diagnose and teach.