Generally agree with all the posts in this thread. I live in AMS and am regualarly down in Paris (and a DLP Annual Pass holder) if you have any nitty gritty questions I can help with. I won't jump in to much already said as I think it's all good guidance.
The one thing I'll add is that I would consider the public bus between MVF and the Marne la Valee train station as the "back-up". Marriott's offered shuttle is only €3 roundtrip per person and much more convenient than the bus (particularly if you're not in the Giverny section of the resort...). The only downside to it is that it doesn't run for about 3 hours mid-day. In the past I was a big proponent of the public bus because the Marriott bus was more expensive (at least this was my memory from early trips), but in my recent trips I've found the bus a little less predictable (meaning I've missed it because it's come early....or found myself going out early and waiting, which wouldn't be ideal in Dec weather). I think it's worth the cost even if you've already bought the NAVIGO, particularly for days you're going to the parks.
And, the Navigo covers all the Paris metros, RER, and bus system, but I'd need a different ticket to take the fast train to Strasbourg right?
Correct, the NAVIGO will not cover long distance trains (TGV or OUIGO).
We are debating in the family right now; the wife and son want to go do a two day one night trip to Normandy whereas the daughter and I would rather add a couple nights at a hotel in Paris.
The only thing I'll say here is that while Normandy is a lovely trip, late Dec through Feb are really the last times of the year I'd ever dream of doing the trip. The weather is just too miserable/unpredictable. The nice thing about the 7 days in the middle of your plan is you can "manage" your outings and activities on the worst weather days as they emerge. If you committ to the trip to Normandy, you're stuck no matter how the weather. If you opt for Paris, the city will offer many more options to avoid the worst of the weather. (I hope that makes sense).
