Well, I fall back on my long-standing projection that once HFR is moved to the Havelock line, VIA's use of the Kingston route will face even more constraints beyond what's there today.
That brings up a very good point. From CN's perspective, separate trains from Toronto to Kingston and Kingston to Montreal or Ottawa would be more disruptive than a single train from Toronto to Montreal or Ottawa (on lakeshore via Kingston), so it might be easier for VIA to have one train along the entire route than two trains that terminate/originate in Kingston.
I am assuming something like 4-6 departures per day to Toronto, Ottawa and Montreal each.
Kingston Mayor Bryan Paterson tweeted this map in 2017 about post HFR frequency of service in Kingston. It shows 12 trains a day to Toronto and 6 trains to each of Ottawa and Montreal. That is down slightly from the pre COVID service of 17 westbound and 13 eastbound trains on weekdays.
Assuming the fleet is sized for the 1-2 peak departures, I would bet around needing 3 coaches (2 economy + 1 business). I'll leave it the more knowledgeable to judge whether that is best delivered by DMU or a loco pulled train.
That is about what I would expect. Given that 3 cars is about the break even point for DMUs vs conventional trains, I don't see the advantage of introducing a new train type into VIA's fleet.
Also, given that most modern DMUs come in fixed, preconfigured train lengths, with permanent (not semi-permanent) couplers, they provide much less operational flexibility to adjust for seasonal demand or long term growth.