Although I have not personally done so, some FT members have called in and successfully changed bookings on the lattitude passes after booking a less than desireable route online. However, I am not sure how this would work with the Sun Pass, since there is a change fee associated with it even though the booking goes into U fare. As such, the pass is an odd hybrid, since a normal U booking would not be subject to change fees. If you do attempt to make the change via phone, let us know how it goes
Although it isn't really my business, I am not at all clear on why you would prefer to route through Montreal unless you needed a couple hundred additional Q miles, since it means your first leg will be in economy class rather than business class YOW --> YYZ. I know changeng terminals in YYZ can be a bit of a nuisance (especially with family), but it isn't exactly a breeze via YUL either (long walk, crowded U.S. immigration area). Also, if you needed the miles I would tend to route through YYC instead.
Back on topic... route availability via the passess can be difficult to get used to. In some cases, I have been presented with connection options that aren't even available via AC.COM, but in general there are far fewer options. First and foremost, any UA codeshares are absent, but AC also limits the number of AC connection options. In the example you provided, I can start in YYZ and route through YUL to LAS even though you are currently having trouble booking YOW --> YUL --> LAS. Also of note is that route availability tends to drift a little from day to day or month to month (odd) and that sometimes you need to play with the flight time options to get what you want, and sometimes you need to ensure that the "All" time is selected. It can be a bit frustrating on occassion if you have very particular flight requirements.
SG