"the ideal shoulder/hip/heel alignment"
There isn't one. It depends on the saddle and what you want to do with the horse.
Equitation is judged in an arena, using horses that aren't trying to cover ground fast. So equitation rules are designed for what works well for that environment and that goal.
On a trail, you'll get the best results by balancing WITH the horse. That will vary with what the horse needs to do. It might be forward, or way to the rear, or somewhere in between.
Based on what makes my horses happiest (most eager, best speed and duration), a forward seat is best for covering ground. That may be because my lower back is too stiff to move the way Craig Cameron does in the video above.
Here is a video of a jumper. Notice how much his lower back and hips move catering with his seat in the saddle. I am physically incapable of moving like this, although I'd dearly love to be able to ride 1/20th that well someday: Rio Grande TROTS a Grand Prix Jump! - YouTube
Western riding isn't about riding in an arena. That is why western riding doesn't have a 'correct' position. Cutters, reiners & ropers and barrel racers all adapt their approach to the needs of their arena sport. And riding isn't about 'position', it is about 'movement'.
Having the back of your heel in front of your belt buckle isn't evil. The cavalry WANTED that, and they rode their horses much greater distances in a week than almost any modern recreational rider...including up to 100 miles/day. 40+ miles/day for 5 days straight wasn't unusual at all.
If you want your heel under your hip, that is OK too - just buy and use a saddle that is meant to put you in that position. If your center of gravity, on average, is centered on how the saddle tree distributes weight, and you move with the horse's motion and balance, then the details are irrelevant to the horse. But it IS important to look at how the saddle distributes weight. English and western saddles have a very different design, and aren't meant to be ridden identically.