"I would never want to exclude her from the rest of the Christmas celebrations, including opening the main presents." IMHO, there's absolutely nothing wrong with her being "excluded" from opening the main presents. She had her time with her children opening theirs, now it's your turn. I know that's not your plan but you still wouldn't be being unreasonable to insist she didn't arrive until 12pm or something, maybe with the presents she's bought so it's about her seeing the kids open them instead of all of them. Also, why the pressure to film your kids so you can send her a video if she's not there at the crack of dawn?? You should be allowed to enjoy a few hours on Christmas morning with your kids and DH without thinking of anyone else or being pressured to film it all.
Of course, it would be cruel to exclude her if she's always been a part of it but I don't understand this new wave of entitlement from grandparents who want to be a part of every little moment so parents and their children rarely get their own little family time and memories that's just them. I'm not saying to exclude her, before people jump on me making assumptions; I just find it crazy so many in-laws and grandparents expect to be included in so much more now and parents often have to just put up and shut up.
I adore my grandma but growing up, we had Christmas morning at home just my parents and sibling, then went to my grandparents' for dinner where my aunties, uncles and cousins would also be there. She never saw us open our presents, she only saw us open the presents she had got us. She never expected more, either. My partner's Christmas Day has always only ever been him, his parents and siblings (and now their other halves, depending on the year as most of us alternate); no other relatives, and it's the same now. He would see grandparents on Boxing Day. People do Christmas different and yours clearly isn't working for you. You're not unreasonable for wanting to change it up and have a bit of flexibility.
As others have said, arriving to someone's house at 6am is absolutely wild and unreasonable - Christmas or not. I'd want to wake up on my own time, have no pressure to entertain, and let the kids sort of rule the timings. Her showing up wanting breakfast and a cup of tea at 6am is too much.
Please stand your ground. 9am is still more than reasonable (I'd be pushing for 12pm personally haha). She's bang out of order to expect an entire family to get up at 6am bc that's what she wants and what works for her. Why are you expected to never have Christmas Day the way you'd like? You've done things her way and you don't like it. DH needs to back you up 100%. 9am is still early!!
Sorry for rambling, I'm rubbish at being concise.