1st Question: I myself have never used the discounts through Costco, though I have purchased an AP from Costco, My dad bought a DLR vacation package through Costco once in the past, he had no issues with it, check into it before buying it: IE: What comes with the package, what hotel is included in the package if one is in the package deal, etc. So if it saves some money, what not go for it? But as Matt said above, Disney is offering some really great deals currently, and sometimes you can find better options through a Authorized Disney Vacation Planner, give Mouseketrips a shot, they offer some great deals and will work one-on-one with you to customize your vacation with what you want and they work directly with Disney, un-like costco which has these pre-planned options where you dont really get to work with disney hand-on-hand and you cant customize things that work best for you.
2nd Question: Crowds at DLR during the Holiday seasons are generally always busy, the last 2 weeks of December are the busiest since thats when the SoCal/California schools are out for Winter Break and gets busier as the actual day of Christmas nears, so the beginning of December, usually in mid-week (tuesday-friday) are the best days to go, weekends are obviously going to be the busiest times of the week. And un-like WDW, At DLR all the Holiday events are free and you do not have to pay extra, The Holiday Fireworks, Shows, Parades, Events etc are all free, there is no special extra-cost ticket to see the Holiday Shows.
3rd Question: My favorite Disneyland Resort hotel is Disney’s Grand Californian Hands down, no doubt about it. The Hotel is beautiful, was designed by Disney, it has a Private entrance into Disney’s California Adventure Park, an entrance into DTD, the lobby alone is amazing, it has on-site restaurants, shops, lounges, childrens work-shops & play areas, story telling, great pools, it now has DVC units on-site, amazing views…it is a wonderful hotel and well worth the money.
*The Disneyland Hotel is great too, but is currently going through a multi-year complete remodel and renovation, the company is doing one tower at a time, then all of the hotels ground and amenities, with a possibly 4th tower being added in the future, it only has a DTD entrance, no direct private park entrance. *Disney’s Paradise Pier Hotel is nice too, its the furthest away with no direct entrance into DTD or the parks, a bit of a walk but still a nicely Paradise Pier themed hotel, if you have the money, stay at the Grand Californian, its really worth every penny…for as much as the hotel offers, its awesome
Pricing for the current 3 hotels is as follows:
Grand Californian: $$$$
Disneyland Hotel: $$$
Paradise Pier Hotel: $$
Good Neighbor Hotels (off-site hotels) are just as nice (most) and are usually less-expensive than the resort hotels, they obviously dont come close to offering any of the amenities or service that you would find staying at a Disney resort, be careful which ones you choose to stay at if taking that option, some charge just as much nightly as a Disney hotel and aren’t always the closest, Stay at a Disney on-site hotel if you can, its just so much more worth it.
4th Question: The finances, Im not sure what to say there, that is something that only you know the answer to inside your heart. Memories like a Disney trip with the family are some of the ones we hold onto forever, but if there are other things that need to be taken care of first, then maybe they need to be attended to first, but if you think you can afford it and you all want to do this (which I know you guys would have a wonderfully magical time) then I would say go for it, make some last magical memories and enjoy the time at the resort.
The Disneyland Resort during the holidays is one of the most magical times at the resort, its so wonderful, I get goosebumps just thinking about it as I type this.
Good luck with what you decide, my heart goes out to your moms parents and best of luck with the homes mold problem.