Is it confirmed that they will be running Swansea - Manchester?
Given that the proposed Swansea - Manchester isn't actually the start/end route, it's just the attaching/detaching point for one of the units which is where they will be 5 car to/from.
If it is Swansea then that makes crewing it more difficult as well given that Carmarthen train crew do a fair bit of the Cardiff - West services.
Also how will the Milford Haven services run? Part of the Franchise requirement is a certain number of direct Cardiff - Milford Haven services, which will be reduced if the Loco's take over to Swansea. That will mean changing other Cardiff - West Wales services, possibly extending some of the Cardiff - Swansea services. Seems a bigger change and more complicated than people might be thinking.
Given that one of the Loco's used to run peak Holyhead - Manchester services it would make more sense to use them on the North Wales coast. It can be confined to Chester, Junction and Holyhead train crew rather than involving Crewe and Shrewsbury.
Also surely any spare loco could be stored at DB Crewe or LNWR which is where they are normally based?
For simplicity it would make sense to confine the loco's to the routes they used to operate, just in higher frequency.
Indeed though whilst @Cardiff123 got the wrong end of the stick regarding the SV vehicles (as they're standard not first class) it is a fair point that the originally proposed formations (assuming that @43096's source pans out!) do have less capacity in standard class than a 3-car 175. From what I can find a 3-car 175 is around 190 seats whilst the proposed Mk4 sets would have 170 seats (with four additional tip up seats in the wheelchair spaces). Of course that is a fair bit more than the 2-car 175s and overall there are more seats on the train as you gain the 40-odd first class seats as well (so total seats are somewhere north of 200). But it is fair to say that standard class passengers lose out.
From the fleet information I have there's 220 Standard class seats and 19 First class seats. The Mk3's had 210 standard and 18 First. By comparison a 2 car 175 has 118 seats and 3 car 186 seats.
In my disjointed experience, it didn't get busy.
I've never seen more than a handful of diners on the southbound service, can't say for the northbound.
Seated premier class diners were supplemented by a steady flow of take-aways from standard (very good value).
The overall passenger numbers were typically less than 50 southbound, peaking into Chester and then Shrewsbury.
Northbound in the Cardiff peak would be very different, of course, as far as Hereford at least, then very quiet.
Which perfectly explains what some don't seem to want to believe about the type of passengers that TfW carry. Lots of regional commuter passengers, more so than long distance end to end passengers.