Actually, good news, I found you an alternate return on 23rd

LAX-LAS-YUL-YTZ

UA452 8:00-9:12
AC576 11:05-18:57
AC7529 20:30-21:40

Continental.com does not recognize YTZ Island Airport as a code, so it wont work, you need to call.

It will be a long day, but you can change your routing if something better comes up for free up to 21 days before departure.

There is no difference if you book one-ways seperately, but you will get charged phone fee if its 2 tickets if you do not book online.