I have ridden the Canadian regularly from Vancouver to Toronto for the last 5 years or so. I have done what you are planning, riding the Maple Leaf the next morning, several times.
First, when is your trip? The schedule is about to change on July 26, so the first departure on the new schedule from Vancouver will be Friday. July 27. The departure time from Vancouver has been moved forward to 12 noon from 8:30 pm and the arrival time in Toronto has been pushed back to 2 pm from 9:30 am. Make sure your connection in Vancouver is still good. My connection, a formerly leisurely 8 hour 45 minute layover from the arrival of the morning Cascades at 11:45 am morning Cascades to the 8:30 pm departure of the Canadian is now cut. I will have to arrive in Vancouver the evening before on future trips.
I feel more confident about the new schedule. Eastbound, it is 12:30 longer than the current schedule, plus I think CN will be a bit better about honoring the new schedule, at least for awhile. They did reasonably well the last time they lengthened it, it has only been in the last couple years that the timekeeping has gotten so spectacularly bad (3 or 4 years ago, we were an hour early into Toronto!). With so many 24-48 hour late trains recently, my original plan for this year's trip was to allow 3 days buffer in Toronto to do what you are doing. Now that they have changed the schedule, I am planning to go back to my normal, one night, layover for my trip in late October.
Bottom line on the Toronto connection, if you are going before July 26 on the current schedule, I think there is a good chance your train will be so late you might miss the next day's Maple Leaf or at least make that night miserable with a middle of the night arrival. If you are going after July 26, I think the next day's Maple Leaf will be fine. I am currently planning to take that same risk. BTW, as far hotels go the Strathcona is about a 3 minute walk to Union Station and is often quite reasonable. There is a link to a significant discount at the Strathcona for Via passengers at the Strathcona on the Via Privilege page.
On my trip last year we were almost 12 hours late into Toronto arriving about 9 pm or so. Still had time for a decent night's sleep and catch the next day's Maple Leaf, but at the rate we were losing time, I was biting my nails for awhile. I wasn't so much afraid of outright missing it, but getting in at 3 or 4 in the morning, then having to get back to Union Station by 7:45 to catch the Maple Leaf.
In a section, you stow your luggage underneath the seat. I don't know how big that is (I get a roomette), but I've seen people in sections with small, airline size carry ons. I wouldn't do the Winnipeg check and recheck because you may be in Winnipeg at zero dark thirty with the timekeeping problems, so you'd have to get up in the middle of the night to do it, and if you detrain in Winnipeg, you don't have access to the train again until they start boarding about 30 minutes before departure. Plus I never had a problem stowing the necessary 4 days worth of clothing in a small duffel (ordinarily my gym bag) that I carry on and that will fit about anywhere. I check my big bag through to Toronto.
With regard to dome access for the Rockies, if you are on the new schedule, head to the dome as soon as you wake up. The scheduled arrival in Jasper is now 8:00 am and the good Rockies scenery is the two hours before arriving Jasper. Also, on the new schedule, I would basically head up to the dome at departure from Vancouver, as you will start through the Fraiser River Canyon at about 2 pm. If on the old schedule, if you are upstairs a couple hours after Kamloops, I think you'll be fine. But I have been riding in the off season, so not sure how mobbed the domes will be. Note that you will NOT have access to the Park car during peak viewing hours. You are right that the domes don't have near the demand in the prairies, although they will remain popular for the next hour or so after Jasper until the mountains peter out.
In the shortest winter consist, 0212 is last Sleeper Plus sleeper. Additional sleepers are added with numbers above it, so position will depend on how long the train is. My guess is in the summer consist you'll be about in the middle. Note that the train in the summer runs almost as two separate trains bashed together, each section has its own diner and Skyline lounge ("Activity Car"). The train will have 4 domes, 1 Skyline for the coach passengers, 2 Skylines for Sleeper Plus, and the Park, mostly for Prestige. You can use either of the 2 Sleeper Plus Skylines, and I imagine you could use the coach Skyline as well, but I wouldn't, just because I'd be taking space from the coach pax, who have enough to contend with.
The stop for the eastbound on the new schedule at Winnipeg is now scheduled for 3 1/2 hours. There is a really, really good burger stand right across the street from the main entrance to Winnipeg station. I don't recall the name, but just go out the front door and you'll see it. However, the dinner coming into Winnipeg is usually the best meal of the trip, they always have offered prime rib as one of the selections for that meal. For me, I'd rather have the prime rib than a burger. However, with the new schedule, I am not sure about how they are going to handle the meals, as major stops (Jasper and Winnipeg) are now square at meal times. Also at Winnipeg, if you go out the "back" entrance, there is the Forks Marketplace, which has a food court. Also, if you bank with Bank of America, you can use ScotiaBank ATMs without out-of-network ATM fees. ScotiaBank locations can be a bit hard to find, but there is a ScotiaBank ATM in the Forks Marketplace.
Note that at the long stops at Jasper and Winnipeg, they close the train to boarding in order to do maintenance. So if you detrain, you will not be able to get back on until they do the boarding call (if you are on, you can stay on, but you won't be able to get off after they close it). At both places, especially Winnipeg, you should have plenty of time. I've wandered reasonably far from the station at both Winnipeg and Jasper. Just be sure and be back about 30 minutes before the announced departure time. They have to do maintenance at both those stops and at Winnipeg the whole crew swaps out, so they are long stops even if the train is late. My last time trip we arrived Winnipeg at 6:30 am and we left at 8:30 am. They'll announce the planned departure time.
A couple of other things. The food is MUCH better than Amtrak's diners and the menu rotates each day, so you are not faced with the same selections day in and day out. The onboard service is better and more consistent. There are little cookies, fruit and tea available for free almost all time in the Park and Skylines. While they sell beverages, the Park and Skylines do not sell snacks. If you want a more substantial snack than the fruit or small cookies, you have to go up to the coach Skyline where you can purchase things like chips and candy bars. There are not the continual PA announcements and "Conductor to the IC" calls that you have on Amtrak LDs. The Via onboard crew uses radio to communicate with each other as well as to the head end. About the only thing they use the PA for is to call meals, so things are much more peaceful. Also, if you have a scanner, it is pretty interesting to listen in on the Via OBS channel.