τὰς φερομένας ἀπορίας κατὰ τοῦ ἐνδεχομένου ἤδη μεμαθήκαμεν, ὅπως αὐτοῦ τὴν άναίρεσιν ποιεῖν ὑπελάμβανον.
We have already learned what difficulties he thinks the abolishment of possibility creates. I am not sure but it looks as if τὰς φερομένας ἀπορίας is the dorect object of both μεμαθήκαμεν and ποιεῖν.
A few pointers: ὅπως…ὑπελάμβανον is an indirect question. The whole clause serves as an object of the main verb μεμαθήκαμεν (we have already learned how…) ὑπελάμβανον is not 3rd person singular(!). τὴν ἀναίρεσιν is the acc direct object of ποιεῖν. At least without more context, I would think αὐτοῦ refers to τὸ ἐνδεχόμενον.
We have already learned how they think that the mentioned difficulties regarding possibility cause its abolition.
This is a prolepsis.
yes