You are correct, there's a whole lot more to "on the bit" then a pretty headset. It's more of a way of going that allows your horse to move using his body freely and and in a way that is balanced. The results are balance, impulsion, engaging his hind end, slight lifting of his back, being soft and submissive to the bit, etc. You are also correct, to start off a green horse, you would first begin by a more long and low headset (it takes a bit to get that higher neck that you see in upper level horses, although, conformation also plays a factor). However, the first picture you showed is NOT what you want! The horse's poll should not be below the withers and more importantly there is NO contact on his mouth! (no submission or acceptance to the bit, that's major). That horse is basically plodding along hanging his head. To progress in dressage your horse needs to learn to shift his weight from his front end to his hind end. That's not what's going on that picture. While you don't want rely on your hands for balance, you must always have contact with their mouth (so there would be a straight light from the bit, through the rein, through your forearm) The lady in the 2nd pic is not balancing on her rein, but she does have a very direct communication. And you are correct, you don't want to be behind the vertical either. There are several causes and reasons. It can make them heavy on the forehand but (purely my opinion, anyone can disagree) I think that can happen more because someone with heavy hands will cause a horse to be behind the vertical AND heavy on its forehand, not necessarily one a result of another. It can also mean that your horse is avoiding the bit. But ultimately, a horse behind the vertical will not be as balanced or soft as a horse that is properly on the bit.
I don't necessarily see training to go on the bit as an A + B + C = D (on the bit) formula of broken up pieces to get to the finish because it all goes together. I see it as more a "a little on the bit", "a little more on the bit", "even more on the bit", etc etc. These are my thoughts on the matter and my personal training principles. Anyone can feel free to disagree as I am NOT claiming to be a dressage expert! I generally avoid this topic because it's hard to explain and more of a feel then a formula or checklist. But insomnia wins tonight!