The question is also whether there is a distinction here between Jewish law and meta-halakhah.

In my opinion, that’s an interesting question.
If someone accepts upon himself the rulings of Maimonides, can he afterward also accept upon himself the second-order authority of a rabbi who is a heretic according to Maimonides’ definition?
It seems to me that he can. After all, in principle one is allowed to follow the lenient ruling of this authority and the lenient ruling of that authority, so one can apparently accept a qualified authority.
That is, if you decided that a certain heretical rabbi is right about this and that, then accept only that from him; and it makes no difference whether it is “heresy” or Jewish law—accept the truth from whoever says it.

To me this is really not interesting. If you decide what makes sense and what to accept (and in my opinion that is always what should be done, if you are capable of it), then you are the rabbi and not he. And this is exactly Rabbi Meir’s approach: he ate the fruit and threw away the peel. In any case, there is no question here of permitted or forbidden.

If someone is not capable, and follows the lenient ruling of this authority and the lenient ruling of that authority without understanding the rulings, isn’t that stringent enough, and all the more so when he decides in an informed way to take one ruling from here and one ruling from there?

I didn’t understand the question. Someone who is not capable is someone who makes decisions without understanding Jewish law. That has nothing to do with leniency or stringency, even if he follows the stringencies of this authority and that authority.