The subject pronoun ce and the demonstrative determiner ce/cette/ces aren't the same word, even if they share a form, and don't follow the same rules.
The subject pronoun is ultimately descended from a Latin neuter singular demonstrative pronoun (hoc), but by Old French it had become uninflecting (ço < ecce hoc).
In Old and Middle French, ce/ço wasn't a ...
In this case "Ce" is correct, it means "cela / ceci" and it's always singular.
You're right, it's a kind of catch all construction.
"Ce n'est pas mon livre" and "Ce ne sont pas mes livres" are both correct.
Actually, I am going to completely disagree with LPH here. This is not a gérondif at all.
These are normal present participles and the en is a pronoun meaning du texte d'Aristote which would otherwise go after parties and sens
If these were gerundives, then there would be no justification whatsoever for having en expliquant and en dégageant, but only suivant ...
No, it is not correct. Here the preposition "en" contributes to the construction called "gérondif" ; it is the equivalent of a subordinate clause (ref.). The subject is "Averroès"; the grammatical fonction is "indication of manner".
You could translate as follows.
Averroès used the method of "commentaire littéral&...