I would not allow doubling back, or multiclass with yourself again. I might, with experienced players, allow access to all the 3rd level paths/specialzations within the class, but you still don't get everything. You get a feature at the appproriate levels and can pick from any/all the paths, but you still only get N features totasl. And if at 11th level a feature improves a 6th level feature you didn't take, will you just limited yourself out of that improvement by not having the prereq. So go ahead and be an Eldritch Battle Master, but you are going to have fewer Superiority Dice and Maneuvers, and Spell Slots/Spells than the purists.
And like I said, I'd only do this with experienced players who had played "everything" already, and only after I did more analysis on it.