You're welcome, Rosy! And thanks for tolerating my writing so much!
IMO, it's beautiful the way you went all out for DS and family, especially for DIL's birthday, after just having had surgery! Some people would have just begged off doing anything special,, saying, "I can't do much..." I don't know if they appreciate that, but I do.
I'm not surprised that DS always arranges the visits - that goes along with the yours/mine idea. Nor am I too surprised that DIL didn't mention your illness on the phone that other time. Maybe she thought expressing happiness that your home was enough. Or that your actual illness was too personal or sensitive an issue. Maybe as you say, she was upset that DH wasn't with her since she was "in the last stages of pregnancy." Unfortunately, she may be blaming you for that, even though it wasn't your fault.
Granted, DS was in a tough position - go to DM who just got home from the hospital and has a birthday coming up or stay with DW who's close to giving birth. Unfortunately, she may feel he chose you over her and the coming baby - very likely a HUGE issue for her - again NOT your fault, but SHE MIGHT be blaming you (sigh).
It IS odd, IMO, that DIL responded so coldly (or so it seems to me) to your efforts for her birthday. Was this before of after DS' visiting you during her pregnancy? if it was after, that could be why, as unfair as that may be.. Or, perhaps you're right, she was hurt by some quip or other you made. Can you think of what it might have been? Has she or DS said/hinted at anything?
"Prior to his daughter being born, he always came home at special times, including Christmas. I told him many times that I thought they should be together, but he said he loved being home. His wife preferred to go to her family, despite my suggesting that they spent Christmas with one of us and the New Year with the other. In fact she was there for several weeks before and after the birth. Now of course my son, quite rightly, spends these times with his daughter, and his wife's family."
I may be all wrong, but it sounds to me as if there were some issues between them before GD came along. Or it may just be that they each had some difficulty letting go of their own side of the family or working out a way to alternate holiday visits. as you suggested. Now that he's a father, it appears that DS has matured enough to prioritize his new core family (himself, DIL and GD) and let go of you and other relatives a bit. But DIL hasn't learned to let go of her side, as yet. Or she won't cuz she's still mad at something you said and/or at DS' leaving while she was pregnant. Much of this is more about them than it is about you ( well, except if one of your comments inadvertently hurt her). So sorry about that.
Then again, it could just be that they live closer to her relatives and that's much easier for them, right now. Maybe as GD gets older they'll begin to alternate the holidays a little more. It may be a good idea to just wait and see.
" I have told him that this is how it has to be. To a certain extent, I'm his past, his little girl is his future."
This I think, is a beautiful attitude! So is your appreciation for DS' love and the fact that he communicates with you as often as he does. I get what you''re saying about emails and texts not be able to "replace" a "hug." But they're better than nothing, yes? If you keep whatever contact you have as pleasant as possible, I doubt you have anything to worry about where he's concerned. Hopefully, in time, that good relationship will include GD.