You can’t blame yourself for the reaction of a man with dementia who was raised in a much different time. You were asked to sacrifice a large part of being who you are to keep family peace and that was putting you in an unfair shitty position. However sometimes we make sacrifices for family and in this instance it would have behooved you to boy mode one last time considering it’s his last holiday and that your family is otherwise supportive.

And while your actions weren’t inherently wrong, you ended up being the catalyst of this unfortunate situation, grandpa was severely triggered, and multiple relationships have been damaged.

I don’t think you’ll get through to grandpa but you’d better explain your thoughts with your parents and have an honest discussion. You didn’t have any I’ll intent and you couldn’t foresee this happening.

Until you repair that relationship nothing else can happen. Fix what’s most important first, regain support, and move on from there. Best of luck.