It seems problem2 was just about adding
def isPublic(self): return False
in my family file : https://www.mediawiki.org/wiki/Manual:Pywikibot/Use_on_third-party_wikis#Bot... Sorry I hadn't found that earlier.
Problem 1 remains :
I'm trying to instantiate a `Page` based on its id, but I can't seem to find something that does… The closest to it seems to be "page_from_repository(pageId)", but I get "pywikibot.exceptions.UnknownExtensionError: Wikibase is not implemented for [mywikifamily:fr]." From there on, I don't know what to do.