Scott, I know this was the agreement, but I don't like it. She seems to be very reluctant about all of this, and she also seems to want to dictate the terms. However, since you did agree to a joint Christmas it would also be bad for you to go back on your word. You are between a rock and a hard place, and this is why agreements like this can come back to haunt you. In the run up to the discussion with the kids, when the idea of Christmas came up you should have punted. "That is a long way off, and a lot can happen between now and then. Let's discuss closer to Christmas." This would have allowed you the wiggle room, to when she started to get difficult, to just pull the plug and say "I think we should do Christmas separate."
Likely you jumped at the idea of joint Christmas as much for your own sake (if you are being completely honest) as you did for the kids sake. I am not questioning your motives, I am simply stating that her being agreeable to one last Christmas together was probably something you deep down wanted (we've all been there), even though it probably was not the best choice to make.
Now you are locked into the "why is it assumed it is at your house?" negotiation. Likely she is pushing this to try to get out of the "sleeping under the same roof" idea you floated. If she can get it at her place, likely you will wait until the kids are in bed, help setup gifts, then go home to sleep to arrive early the next morning. I mean, personally I wouldn't want to sleep at my W's new bachelorette pad.
This is your chance to be assertive.
"I think the kids would enjoy Christmas more in the home they already are familiar with and most comfortable in. Also, it makes the logistics much easier as far as sleeping arrangements. I feel this is the best decision."
"As far as which mass, here is my opinion. 3pm will make it easier to then get dinner, and get the kids down for the night. We can then setup the gifts and settle in for the night."
"If you would prefer wrapping the gifts there I am okay with that. We can work on the logistics later. I will put the gifts and the paper in the garage as you have suggested."
You are still too worried about what she thinks. "she will chalk it up to me "gaslighting" and manipulating her again." WHO GIVES A CRAP WHAT SHE CHALKS IT UP TOO??? Stop focusing on her, her feelings. She is not your responsibility anymore, remember? She fired you as her H! Stop trying to act like you still have that job.
M(53), W(54),D(19) M-23, T-25 Bomb Drop - Dec.23, 2017 Ring and Piecing since March 2018