Call AC and ask them to hold an itinerary for you. That way you'll also see if it affects the Aeroplan inventory.

But before you call, see if you can get a similar routing through ITA Matrix. If you're mixing a bunch of *A partner and stopovers, you'll have a really hard time finding an Airline that will be able to sell you a ticket at any reasonable price.

What you could do is message people on FT what you want to book and have them pull the fares and fare rules on EF. They'll see if you're allowed stopovers, how many, and for how much. At least you'll get a better idea of what can be sold.

As for combining airlines, no idea. If there's an AC codeshare, then you'll likely be fine, but if you're doing something like YYZFRA on AC and FRANRT on NH, good luck finding an airline that can sell you that at a reasonable price.