Should start 3 days a week on May 5 leaving Toronto, Sunday May 8 leaving Vancouver. Tuesdays from Toronto and Sundays from Vancouver are the days dropped during the winter season, and I did a trial run reservation on www.viarail.ca on those dates and they are open, so the train is running. It is 3 days a week starting the first week of May.
It really does depend on what accomodations you want. Berths and roomettes often go begging. Bedrooms are what book up. If you can live with a section or two roomettes (same price as a bedroom), you almost certainly will get space. Since Via prices fares per person, there isn't a difference between two people traveling in two roomettes versus traveling in one Bedroom. A Bedroom ("cabin for 2" in Via's current, non-traditional parlence) may be iffy in your time frame in late May, but I wouldn't think early May would be a problem. The discounted versus the non-discounted fare isn't an issue for you, apparently. I don't know how that works, so I don't know the willingness of Via releasing space for the travel agent discount when there is significant demand, only you would know that. If it isn't an issue, then you ought to be able to get Sleeper Plus space of some sort.
If you are flexible on accomodations and May dates, I don't think you would have any problems at all. Even if your dates were set, as long as you are flexible in accomodations, I think you'll be all right.
One note, over the last 4 years, I've ridden the Candian 3 times. The last was in winter (March). I discovered I much preferred the train in the off-season when it feels like a real train serving real passengers (8 cars) as opposed to the bloated 26 car tourist behemoth it becomes during the summer. It is a great ride no matter what, though, and the very last of its kind.
I am about to book another trip for March 2016, BTW. I live about 120 miles from Vancouver and about 25 minutes from the Everett station where I can catch the Amtrak Cascades to Vancouver, so getting to the starting point for a trip on the Canadian isn't a big deal for me and I can be relatively casual about getting to Canada to ride it. I want to keep my 50% off discount for being Via Privilege level coming so I can keep riding it every year