First, you won't have to worry about being shut out of the parks. Typically the only park that reaches phase closure is MK on Christmas Day itself, and even then your odds of being shut out are really small. You'll be fine.
I will disagree with the previous posts and say that yes, you will be able to get FP+ at 30 days out. We are an offsite only family and have done Xmas week twice. No, you won't get FOP, but pretty much anything else is possible. Plus, if you learn and understand how to refresh, you can get just about anything you want the day of. As has been said in this thread, lines during Xmas week get crazy long. There are even lines for womens' bathrooms (felt bad for DW and DD11), so if you don't have a FP+ or know how to get one, you're going to be waiting in lines. However, we never waited for anything and got a good selection of attractions every day...FOP was the only one we didn't get.