Oh Jesse, you are really going through it! and to have so much emotional trauma in the run up to scary surgery, you could just do without it! However in some ways it may be a relief. You will not now be on edge post surgery worrying that you are a burden to your partner, or if you are expecting too much...he has already let you down, so now maybe you will be able to relax and let grandma take the strain. Grandma is maybe 73 years young! and speaking as a new grandma myself, I think I would be glad to be the main supporter of a grandchild and to really be of use. You may well find that grandma moves up a gear! She won't be worrying about overstepping the mark (or her welcome) , by stepping on the toes of your partner, he will be out of the way (relationship wise anyway) and you say already that she is your greatest supporter, so I bet she will enjoy her new role as main carer. At 73 she will be retired right?
Of course you will feel grief and loss...how could you not? but ultimately losing you is his loss..you deserve better!
Funnily enough I have just last week been offered 29th July too! I saw my surgeon last Wednesday, and he has put me down for Bilateral surgery too, but PKR instead of TKR, as it is only the medial aspects of my knees that are affected, well at least for now. I deferred , and did not take up the offer of the 29th as we are on vacation then, and I didn't want to spoil it for my husband, or me! How could I enjoy a vacation coming straight back to that!? So I was told that they will be scheduling August about now and I will be top of the list, so I am expecting a letter any day now for an early date! Then like you the collywobbles and nerves will start, when the countdown begins!