"The two challenges I have are assumed knowledge and lack of skills.
I've read the books, watched the videos, read the codes, helped my handyman friends. There's no substitute for DIY."
I guess I disagree.
Without trying to sound like an elitist, my father taught me that doing this kind of work is not "brainy" work, but it is labor intensive and requires careful thought. With no offense meant (really!), it sounds like you are skipping the last part :)
Every book I have tells you to make sure header boards are true. Nailing schedules are often controlled by building code (though maybe not in this exact case, dunno where this closet is :P). Unless you are building a house from scratch, a framing nail gun is probably overkill, etc.
These things you call "learning mistakes" sound like "not being careful" to me. I see this often with software dev friends I helped. Because they are so used to throwing stuff out, they almost try to "incrementally" frame a window or whatever until they get one that works, rather than carefully think about where they are putting nails, measuring, marking, leveling, etc, before doing anything.
Remember that the contractors and handymen you watch have done this all a long time, and can probably do things like simply look at a frame and tell whether it's true with high accuracy.
They also don't need to think as much about how to put nails in.
Again, i'm really not trying to tell you "you are doing it all wrong", just offering an opinion :)