Problem #1: the Quebec Act only really formalized what was already happening in Canada in the first place, side-stepping the law in order to keep things moving because of obvious demographic reasons and the differences between French colonial law and English common law. That and the British wanted to create a conservative elite that would naturally look to the British for protection, much like what already existed in the UK and the 13 anyway (in fact, I'd argue the model for this was Ireland, but with the Catholic Church in place of the C of E because it was clear to everyone that the Francophones were not going to convert en masse and the Catholic Church was struggling to maintain relevance in Francophone Canada, operating in survival mode after the Conquest). Some form of a Quebec Act was going to happen anyway, even if not exactly like the OTL Act, but TBH even then, no Quebec Act really doesn't change much of what is happening. At the time, this wasn't really seen as oppression per se, because various elements of the British military were doing their best to win over the hearts and minds of the people, but as the price for doing business. We have a conception in the popular American narrative of US history that Canada was a pretty tyrannical place in the immediate aftermath of the Conquest, compounded by the Quebec Act as both a last-ditch concession and as arbitrary legislation that deprived Quebec of a proper government and a common legal system in English law, projecting onto Canada all the same afflictions that affected the US for over a century at that point - not helped by French-Canadian nationalists later on using a similar narrative of oppression for very different conclusions - but the reality in pre-Quebec Act Canada was very different from what that narrative suggests. A Canadien living in Canada before and after the Quebec Act was passed would not have noticed much of a difference (well, not in a manner that Americans would recognize - the seigneural system's return was a bit of a sore point, but at the time no one in the 13 was going to understand that, not only because similar mechanisms were already in place in the South but in order to have France as an ally, they basically had to overlook what was actually happening there since the rebels were desperate for any help they could get). True, there had been attempts early on to assimilate the Canadiens into the British milieu, but those proved to be unworkable very fast - hence the compromises that ultimately made the Quebec Act possible.
The Canadiens are not going to revolt, if they do at all, to the same degree that the rest of the 13 did because the different colonial experiences are different. Plus, even if Canadiens were non-practicing, the 13 in general were really leery of taking on a bunch of Francophone Catholics - because it hits way too many sensitive spots for Americans, not just the France-England rivalry (compounded by the French themselves actually being the enemy in numerous wars, most recently the Seven Years/French and Indian War from which the Conquest comes from) but also because of a long strain of anti-Catholicism that dates back to the Reformation and the formation of the C of E in the first place - and it's that lack of a common language that seriously hampered the efforts of the rebels to communicate with both the Canadiens and the occupation of lower Canada early on in the Revolution. So I could see Canada's effort going as well as OTL, or even worse for the rebels in part because the only ones that would have really revolted would have actually been a small minority mainly among transplants from the 13 - with the Canadiens indifferent/neutral to what was happening and/or even actively hostile against the rebels because the rebels are perceived as being insensitive - what worked elsewhere was not going to translate well in Canada.
At the same time, I don't see Canada seriously considering going back to French colonial rule - there was too much bad blood between the Palace and the Canadiens themselves to consider restarting that enterprise (not to mention that Canada was a money sink and unprofitable for the French - Haiti, OTOH, as well as various other plantation colonies elsewhere, were profitable and hence more likely to retain French colonial rule). That France became a US ally at all is actually pretty remarkable, but it should be noted that nowhere in all of that did the French show any interest of taking Canada back. If the Canadiens do revolt, then, it would be against the rebels (for basically mistreating them) and either bring back British colonial authority or find some sort of neutral status that would still be very favorable for the British. So while it would not be strictly pro-British, I could see why it would be perceived as such by both the rebels and London. The status quo, while not great (and could definitely use reform), was working fine - so what's the point of rebellion?
Problem #2: Newfoundland was simply not interested much in terms of what was happening on the Mainland. While they know that it's happening, the island is simply too far away to make any real difference anyway - not to mention that Newfoundlanders were far more interested in what was happening in Britain instead. When you consider that permanent settlement of the island was only legalized at that time very recently, and hence most immigrants were people who still had family connections back in Ireland and southwestern England, then it's easier to see why they were not too terribly concerned with what the rebels were up to. Also, Newfoundland does not share the same grievances that the 13 did with the Board of Trade, the Colonial Office, and the like - because Newfoundland, for a long time, had a very different relationship through the Admiralty with the Board of Trade, as Newfoundland was technically a fishery and not really a colony. Permanent settlement was only really legalized so that there would be, essentially, support staff for the Grand Banks fishery operations. So, once again, we hit a problem in terms of lack of interest from the people actually living in a place WRT the Revolution.
Now, I'm not suggesting that the whole premise of the OP is completely ASB. What I am suggesting is that, when the reality of both Canada and Newfoundland are taken into account, it's hard for me to come up with a scenario where both could join in - and for Canada in particular, the few issues that are salient for Canadiens are not that salient for the 13, so it's like the Canadiens and the Americans are talking past each other. Plus, in times of uncertainty and instability, it's always a good idea to keep one's head down and avoid controversy where possible.
