You have to read the detailed fare rules out of YYZ (best seen on Expedia's site).

I doubt you can get a cheap fare from YYZ via YYC and YVR. The routing rules for your fare are (on AC ticket):

FROM-TO YTO-AC-MEX*
FROM YTO-AC/CP-
TO MEX*
OR TO CHI-AC-MEX*
OR TO NYC-AC-MEX*

or on MX ticket:

FROM-TO YTO-AC-CHI-MX-MEX*
FROM-TO YTO-UA-CHI-MX-MEX*
FROM-TO YTO-AA-MIA-MX-CUN-MX-MEX*
FROM-TO YTO-AA-NYC-MX-CUN-MX-MEX*
FROM-TO YTO-CP-NYC-MX-CUN-MX-MEX*
FROM-TO YTO-UA-DEN-MX-MEX*
FROM-TO YTO-AA-MIA-MX-MEX*
FROM-TO YTO-AA-MIA-MX-MID-MX-MEX*
FROM-TO YTO-AA-NYC-MX-MEX*
FROM-TO YTO-CP-NYC-MX-MEX*
FROM-TO YTO-AC-YMQ-MX-MEX*
FROM-TO YTO-UA-CHI-MX-ZCL-MX-MEX*
FROM-TO YTO-MX-MEX*
FROM-TO YTO-MX/AC-CHI-MX-MEX*
FROM YTO-MX/AC-
TO YMQ/EWR/DEN-MX-MEX*
OR TO MIA-MX-MEX*
OR TO MIA-MX-CUN/MID-MX-MEX*
FROM YTO-MX/AC-
TO CHI/EWR-MX/AC-YMQ-MX-MEX*
OR TO YMQ-MX/AC-CHI/EWR-MX-MEX*
FROM-TO YTO-MX/AC-DEN-MX-MZT/PVR/SJD/
ZCL-MX-MEX*
FROM YTO-MX/AC-

However, you can get "cheap fares" on Mexicana (using AC and MX flights of course) from YXU or YOW (for about $1800 and $1600 in J respectively) which can be flown via YYC. If you also want to add YVR it will cost you more (exceeds MPM I believe). Better to go YYC-LAX direct.

Yes your 1398 fare has unlimited stopovers but they would have to be DEN, NYC, MIA, CUN depending whether you were ticketing via MX, AC or UA (see above). Other cities are not on the permitted routing. Montreal is but that would make for a rather expensive YYZ-YUL trip.

The rules are not that complicated but you have to fool around with them. I would recommend using a more powerful booking engine like ITN (allows up to 8 segments) to do your testing of fares and routing and then either purchase on ITN or go to a travel agent.

I will not/cannot book them easily using Expedia, Travelocity or Air Canada.

I would also recommend going over all those past threads and high-grading out the most relevant info about where to find fare rules, what the different itineraries are, why some cities are not doable (like YYZ--you have to go to YXU or YOW to start your trip) etc.