Thank you all for the prompt feedback; it proved to be the encouragement I needed to maintain my confidence.

It took 45 minutes, but I ultimately prevailed. First I was told the routing was simply unacceptable as LAS was not on the most direct route back to Canada. I pressed the agent on this point for several reasons, and ultimately she said she was certain it could not be done but would check with a superior. While "waiting on hold" for the supervisor herself, the agent then suggested that there was not availability between SFO and LAS even if it were possible. I stressed that I, in fact, saw availability and gave her the flight numbers. After speaking to the supervisor, she said that my flight already had "too many" stopovers. Apparently, the overnight layover I had no choice but to accept for my award on the outbound seemed to qualify as a "stopover." I emphasized that this was hardly my preference and that I would be happy to do away with my "stopover" night in LA for a same-day trip to Hawaii and instead go ahead with my "stopover" in LAS as desired. Same-day availability to Hawaii did not exist, of course, so she let that go. The return connection in SFO was similarly long, and again out of Continental's "availability" necessity, not my preference. Finally, when I assumed all hope was lost, she said, "Alright, I'm going to do this, but you cannot change your layover in SFO to be any later than it is before going to LAS." As wasting hours in SFO is not on my ideal vacation activity shortlist, I affirmed that this will likely not prove to be an issue.

Interestingly, she had no problem changing my outbound flight to originate in Montreal. I point that out as curious simply because I asked for a connecting Air Canada flight from Montreal to connect to my original flight from Toronto to LA (selfishly, so I can meet my spouse in Toronto for the trip). Per the agents strict adherence to the "most direct routing" rules, I'm surprised I got away with it so easily, particularly as YUL-YYZ-LAX didn't even show up as available; I simply piecemealed the flights together myself. Maybe I was so successful because we did this first. (Though, as Channa recommended, I explained my whole purpose first -- though naturally, it was not until we started to do it that it suddenly became impossible.)

Anyway - success in the end, with as few fees as possible, and a trip to see the folks after a trip to Hawaii; hardly anything to complain about. Thanks all. Happy New Year!