Originally Posted by canadiancow

I would not assume that.

For one of my EYW itineraries, it has 4 fare components, one of which is J, on which only one of the three segments even has a J cabin. Only that fare component entitles me to MLL access.

Imagine the following, all on one ticket, all on one day:
YYZ-YWG Y
YWG-YUL Y
YUL-YVR J
YVR-YWG Y

I absolutely do not have MLL access in YYZ, YWG, or YVR.

We're not talking about YOW-YUL-LHR where it's one J fare and a single-cabin segment.