#1 isn't going to happen because NIMBY's.
#2 isn't a factor because in no way, shape, or form have they even begun to exhaust what service diesel can provide. That's a wretched schedule with very short trains. And besides, the number of passing sidings that would have to be installed to achieve service anyone would call reasonably dense so greatly outstrips the choice of rolling stock that they've got plenty to do.
#3...I would say, much like SLE, that the demands on the equipment pool for branchline service are such a drop in the bucket it doesn't functionally matter. The MTA hates that SLE is going to vulture 23 or whatever M8's...and that's why it's still all these years later being hashed out. But that's largely because they're the ones pushing Penn Station Access as their #1 Metro North goal, while CDOT has intrastate expansion goals. It's a political turf war. In the real world branchline service just isn't a statistically significant portion of equipment demands. If there has to be an "M8A" supplemental order of exact lookalikes in 8 years it's PSA service alone that's going to force it.
But...I'd also say maintenance of the diesel fleet is in the same boat. Danbury puts so little demand on the systemwide fleet that any notion of whether operating mixed diesel and EMU through South Norwalk is sustainable is way overblown. There is nothing you can throw at Danbury that Stamford shops won't be able to handle. Total non-issue for choice of mode.
#4 gets answered when they replace the P32's. The next duals fleet is going to be far, far larger than this one. Hudson Line PSA demands it, and it'll be a combo order with LIRR's for one make replacing 31 P32's and 21 DM30AC's + padding all in one shot. They can afford to pack that fleet padding to the max with the scale of the order sharply reducing their unit price. In fact, with only 12 BL20GH's to go around the diesel shuttle fleet is going to be such a small outlier compared to the probable number of next-gen duals available that I could see the MTA simply transacting its 6 Brookvilles over to CDOT and going 'clean' with one single make for all locos in the MTA-owned pool. The Brookvilles could become an entirely CDOT-landlocked feature. And as for the coaches...with the Shoreliners getting 100% displaced for MLV's and the bi-levels being so badly needed on the Hudson and Harlem vs. in CDOT-land, intra-agency transactions of best-of-the-rest Shoreliner dispersals become so cheap and easy the CT branchlines will basically be able to get the fleet of their dreams (well...numbers-wise at least) for pennies on the dollar. All of this is a non-issue the way it times with the huge slate of 2020 power and coach procurements.
A little urgency in the push for New Milford is going to be a good thing. Malloy's humongous transportation bill includes yet another stab at extending 'Super 7' a couple more miles north. Which, given how many times that's been torpedoed before almost makes the New Milford commuter rail extension a necessary prerequisite to touching the road again. Agreed...they don't have the money today, but the whole 'Super 7' debate by its lonesome is tortured enough to light a major fire under the rail extension as a top priority after ongoing capital projects like NHHS are closed out. CDOT may be CDOT, but I think this one is a reliable bet to get real on the planning front within 5 years and be under construction in the next 8-10 years. Route 7 is just that much the third rail of Fairfield County politics that I can't see that one proceeding to design until they have FIRM re-projections on traffic volumes that account for any and all mode share diverted to a maturely-developed New Milford commuter rail service. The road expansion opponents are just going to tell them again and again to refine their capacity assumptions first, and that's how the commuter rail extension and schedule expansion becomes the default prerequisite and cost of doing business for the Dept. of Asphalt on this ever-controversial stretch of asphalt.