Good grief. What's STM ridership like after Covid? What's different (other than the current service levels) in Montreal compared to here?
I didn't find the Rigaud/Hudson line difficult when I used to ride it counterpeak and to get to West Island on weekends, 35 years ago. Yeah, you had to make sure you caught the rare train ... or risk schlepping downtown to get the 211.
The other thing to consider that I didn't mention before, is something I might call "GO Bus 18 syndrome", its when you have a parallel bus route that's actually faster and more convenient than your rail service, that using your rail service is seen as a detriment. For what it's worth, even though Exo rail is in a massive slump and is struggling to regain ridership partly due to its antiquated 9-5 business model, exo busses are doing better than ever, especially bus routes that duplicate rail services (Mont St-Hillaire is probably the best example of this). It will probably take a lot of money to upgrade and refurbish these lines to make them competitive against their busses (especially off peak), and that's money I don't imagine the ARTM wants to spend.
You kind of have to wonder if the REM has done more damage than good? Does it still prevent the Mascouche line from going through the Mount Royale tunnel? Does the train still have to take the 30 minute detour between the CN & CP yard?
REM has permanently prevented Gare Centrale from becoming Montreal's version of Toronto Union station.
Is the operating cost for the REM higher than the EXO line it replaced? Could the money for REM not have been better spent improving EXO?
What was wrong with the DM line that the city figured they should replace it with the REM?
So the answer to this question really depends on whether you're taking a holistic/idealistic view of transit planning in Montreal, or you're taking a more pragmatic view of the state of transit planning back then (and frankly, today as well).
If idealistic, then the answer is... probably not? In an ideal world the Mont Royal tunnel is preserved as a major piece of infrastructure to allow for through service in Gare Centrale to be used by several new lines including separate Mascouche and Repentigny Lines, a double tracked DM line, as well as used as a through station for VIA HFR. However, that's not the world we live in.
The world we live in is one where the ARTM and its predecessor the AMT are absolutely awful at getting anything done, where transit plans have a life span of no more than a year, and even something like refurbishing the DM line was a political quagmire. The reason why the REM exists at all is because it wasn't planned or operated by the AMT/ARTM, and was instead a "private" venture by the CDPQ (as private as a provincially run pension can be). In terms of origins, the goal of the line was to connect to the south shore, west island, and airports, however the CDPQ also asked to hand over access to DM and Mont Royal Tunnel because there was a concern they couldn't get enough ridership with the west islands and Airport alone, (let alone justify the cost of building a brand new tunnel), so in the absence of any other plans, reusing the DM line for a project like the REM is frankly a no brainer. Being realistic, it was either the REM or nothing at all, and in that context, the REM is an absolutely amazing project.
In terms of answering your specific questions:
1) No, but the Mascouche trains won't be travelling around the mountain to reach GC either. Instead the trains will be terminating at the Cote-de-Liesse REM station, not exactly a cross platform tranfer... well ok it technically is but only to (I believe) the northbound platform.
2) "Permanently" is a harsh term. In theory there's nothing stopping Montreal from building a 2nd tunnel, probably travelling from GC to Parc. This would allow for similar improvements as before, such as direct rail access to Repentigny and Mascouche, while also allowing SJ line trains to go directly to GC as well (and through running HFR trains). The question is when and for how much.
3) Almost certainly not.
4) Someone actually had the money to build it.