Spoilers for HPMoR (though I guess that’s a given at this point):
c) how does she miss the fact that the map shows two Tom Riddle’s walking down the passage way to the graveyard? That should not be easy to miss. It says “it was tangled and confused”, but it would still show the name twice, and if she can read the one, it stands to reason she could read the other. Especially since we have precedent from HPMoR of both being in the same room without the map being confused.
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?