Even if you don't get on the LAX-LAS segment, I'm almost positive the bag will remain on the flight, as bag matching is only done for international flights - not on domestic.

As for short checking, you will probably have to be lucky to find an agent who will do this. Probably more likely if you have a relatively long connection. AC, as I understand, is probably about at least as strict as UA one checking bags, so I wouldn't count on it.

As for me, I was able to short check my bags on UA out of YYZ back in about January, when all those silly carry on restrictions were going on, and they were searching pretty much all carry on luggage. Probably helped that I knew the agent checking me in (and many others, since I am through that station a lot), but I suspect it also helped a lot since I would have been able to carry it on domestically within the US. I had a 2.5 hour layover at ORD, so plenty of time to get the bag and carry it through to my next flight.