Great post, lots of truth, but i'll add something about what you call "muscle memory"
What ethox said
People tend to forget how training muscular and nervous systems work. Studying how a human body learns mechanically is very, maybe almost as important as psychological aspects of a self-improvement game such as this.
is very true. Knowing how neuromuscular training works is a big help to become good at stuff.
Lots of people are aware of this "muscle memory" thing, without truly understanding it. Those who play instruments are probably the most affected by it. "Do this legato exercise everyday and eventually you'll be able to shred like Eric Johnson".
This is due to a phenomenon called neuroplasticity. Basically, if you work on something intensely (by pushing your limits every time) and on a regular basis (3 times a week at least), your brain and nervous system will actually change in order to make you better at what you do. New connection will be formed between the most solicited neurons in your brain and your nervous system will strengthen the connections with the muscles you use.
If you played the guitar for 20 years, you can hear yourself say stuff like "This F chord is so easy, why can't you do it you scrub" to a beginner, but you have to remember that you are literally made to do that F chord. You made yourself that way through training. It's the same with games, you made your brain to be able to see ar10 notes coming, you made yourself able to stream super fast through training. So train by pushing yourself outside of your comfort zone, and slowly make those hostile 80% accuracy songs feel like the tutorial.