Diamond
I don't agree that you're overthinking this but I suspect you have been unable to talk through this complex situation and so it's going round and round in your head.
I do agree with a Christmas card message of "always here for you"
Yes, she's a 26 year old adult but she's also a grieving child inside and that is what you're responding too, quite naturally. Comments that you're just bothered for yourself are shallow. Ignore them.
My perception is that yes DGD can't face coming to your house because it's where her darling granddad died, but also I suspect she is deeply distressed that you do seem to be able to stay there. She may wrongly be subconsciously thinking you aren't missing DH (enough) and her circular private thoughts are negative about you.
Finally, and this is very difficult, she might be pushing you away as a misguided form of self protection. She can't face being close to another person who's going to leave her/die like everyone else she's loved.
Knowing how deeply you love her, I understand why you want to help her, indeed may feel you have an ongoing responsibility to help her.
Sadly, I think there's little you can do beyond the "I'm here" message until she gets help from someone outside the situation.
Where to be interred ? Burial Decisions