I don’t think that these quotes imply that Occlumency is a perfect shield of the mind. IIRC it specifically counters any attempt to divine whether or not someone believes they are telling the truth.
To completely Obliviate someone, you don’t need to read their mind first, you are just setting all the 1 to 0. (Apparently somehow only hitting memory and not vital processes.)
I don’t see a problem then. If you want to find out specific information, you can not use Legilimency, unless you first somehow break the person. And you will never know if you actually did, if they are a perfect Occlumense, so this doesn’t work.
It would have to be tested, what exactly happens if you try to delete something from memory, that isn’t there in the first place. And also what actually happens, when you try to delete a part of the Occlumency Barrier. If I had to guess, though, you would literally just cut a hole in a pretend personality. So if the Occlumense has f.e. 2 layers active plus their “base”, they could have each hold/pretend to hold the knowledge of how to do Occlumency. If you now delete it from the top layer, you will just be let in into the next one, which is being adapted to represent the changes. Also, I would expect the Occlumense to create another layer.
This also assumes that you actually see the whole personality the Occlumense pretends to be, which is not actually correct, IIRC. You only see surface thoughts. Which would probably muddle this even further.… Still, I don’t see a problem there.
Did I miss something obvious?
I enjoy the story, however there’s at least two points here that I can’t just let pass:
How would Finite reawaken an unconscious person? Isn’t it more likely to assume that Stupefy knocks you out, instead of the theory that seems to be put forth here of the hex actually being an enchantment that persists over x amount of time?
A Finite has to be at the same strength or stronger than the spell it is dispelling. I believe that the ring has had several protections put on it, at the end of the first year. A standard Finite, from Lockhart of all people, should not suffice to overpower something that Harry and Moody together thought up. Especially considering that the protections were probably cast by Moody himself, and General Chaos has had to consider Finite in battle for months at that point.