Start doing your homework as its out there. There is no set date other than time frame when EMU and Tier 4 go into service.
I did, in fact I also looked up as to what's involved in a partial or total rebuild of an F59PH to Tier IV. Being done by a number of operators, and some California operators specialize in it. EMD even have a PDF on it. Whether that's cost-effective or not is a very real question, but you contradict the very point you attempt to make: "There is no set date other than time frame when EMU and Tier 4 go into service".
That might just be, to paraphrase you, why "doing my homework" showed no definitive answer. So as a stop-gap measure until the emus become available, that appears to be a very viable option to use them.
As for your 3 trains for your service that takes 28 minutes from Bramalea and 53 from Mount Pleasant, you need 4 trains for 30 minute service, with Mount Pleasant being push to the point it may have to be something different.
You overlook the present extant hourly service from Mt Pleasant in reaching that figure. It could well-be that the present day time service to Mt Pleasant could also be run by three coach consists. I've been on that run every other day past few weeks, it is *grossly under utilized* for even the six-coach consists.
Whether or not it would be worthwhile reconfiguring service on that segment of the corridor to do more runs, with less coaches, is the question, and what stock is available to do it. It certainly appears, from the very information you post, that there's going to be spare loco power.
Again, I propose examining "using what we have" to see if a more frequent and *supplementary to peak time trains* can be run on that corridor. As it stands, that corridor is woefully underutilized while even between rush-hours, the highways are jammed.
The sooner more regular service is run on that corridor, the sooner the intersecting bus connections can feed into it on a 15 min schedule.
Edit to Add: There's a greater issue at play here than just utilizing what Johnny Plebe has already paid for, and continues to pay for out of tax dollars.
When I viewed your pics of the *never ending* Burlington Boondoggle, I was jarred again at the massive amount of money spent on palaces, while Johnny Plebe has to view all of these incredibly expensive expanses while he can neither afford a home or car of his own. He wants to be able to hop on a bus, then hop on a train, and go downtown to work or do whatever. And the Western Corridor is another living, breathing White Elephant.
What is so wrong with trying to find ways, even if temporary, of utilizing spare stock to get that White Elephant moving down the tracks?
What's the deal here? Mr Plebe only gets to look at the train-set he's paid to buy, but not use?
It's time for Metrolinx to maximize the returns on investment. We saw this clearly with UPX, it was a an offer to the Gods of Conspicuous Consumption that Mr Plebe was initially financially forbidden from using. Thus the *fully justified* outrage.
UPX will never pay its way, so why the hesitation and obstinateness of finding ways to make the Corridor work, and make it work soon?