"Why do men's coats and jackets—and shirts—traditionally button left over right"

If you're one of the right-handed majority, it just makes dressing easier. Ladies, though, were dressed by their maidservants, so their coats button the other way.

"...courteous for the man to walk on the outside..."

In certain places it is now preferable for the man to walk on the inside. Muggers lurk in doorways.