McMahon is a city facility as far as I know. Like the Saddledome and the Event's Centre, CSEC is a tenant. You can ask yourself, what say do they have? CSEC would tell you a lot since the facility's success depends on their teams' success. For CSEC you don't have to look that hard to see a number of synergies with bringing both major teams into the C+E. I would argue, you could use the techniques of Judo and use CSEC's strength and size to take on the Calgary Stampede in what I would call a double-team. I actually don't think you really need to use up that much of the grounds space to give everyone what they want:
New stadium where the grandstand is that has open ends to accommodate the chucks during Stampede.
CSEC office space at the Events Centre, that's happening anyways, just include the Stampeder's staff.
Stampeder's practice field in place of the Saddledome, this also gives the Stampede a flat piece of land they can still use during the 10-day event.
I actually don't think Stampeder's would have to be on the road for a month. Perhaps play at home the Friday of weekend before the start of Stampede, then on the road for two weeks and back at home the Sunday after. That gives you a decent buffer, and sure not everything will be completely wide-open but it could work.