As to the shows, I can highly recommend Mystere. I haven't seen "O", but it has a specialized theme focused around water. I actually think Mystere may be a better way to get the full Cirque de Soleil experience. The price for Mystere was about $85 a couple of years ago. (If you decide to go with "O", I don't think you need to pay anywhere near $300 for a good seat.) Some of the LV shows are dark in the first half of December, so be sure to check before you set your travel dates.

As to the hotels, I think you'll get the best prices direct from the web sites. I found that the travel agencies and consolidators usually have higher prices that what you can get direct. Sometime this fall, the hotel/casino web sites will begin to post special rates for December dates (pre-Christmas). One idea for your Mom is to get her a mini-suite at one of the nicer hotels. They are sometimes surprisingly cheap at slow times of the year.

You can sign-up for the player's card online, but without any play that's not going to get you a better deal on a room rate. You can also sign-up online to be sent email promotions and you might snag a special discount code.

Have fun!