I did something very similar. My dm was unwell and would not have been able to attend a wedding. As a consequence of her illness, she would have severe fits, and worrying about the possibility of having fits in public with people (even close family) looking on made her anxious and stressed which made the fitting more likely 
There was no option for the ceremony which would worked (unless it was just dh and I getting married in my dm's house with her and df and witnesses - but I guess that would be more upsetting to the OP!! (and also to her ds)). Even with just the closest of family (parents and siblings) it would have been 15 people and there is no way dm would have coped with that. If we had done that, then the whole day for me and my siblings and my df would have been shaped around looking after my Mum, anxious that she might have a stress-induced fit and not even make it to the ceremony.
And, yes, selfishly on my wedding day, I wanted the event to be about me and him. It was glorious, intimate, personal, and it was about US. Our families are very important to us, I love dmil & dfil, and dsiblingsi-in-law but we were trying to make the best decision in some difficult circumstances. My dm was delighted that we'd got married, and when she died I was really glad that we had done it (and not at all guilty that we had not had a wedding where she had been there). And we were all sad that her health problems had such an impact on her life, and on the decisions we had to make.
There was no way that I would have had a wedding with other guests and not have my dm & df there (df was her carer and would not have left her on her own).
My dm's illness meant that any social occasions were difficult for her (and for other people) and so like the OPs ds, we didn't have a combined family party. Instead we had a series of lovely and intimate mini-celebrations with close friends and family, and then a bit of an impromptu bash with our wider circle of friends.
I was never the kind of person to have a huge fussy wedding, but I guess in an ideal world I'd have had something lovely with about 40 really close friends and family. So when people call me selfish for 'eloping' I have to bite my tongue a bit. The wedding we did have was lovely, but I too missed out on the wedding I might have wanted. I would have loved my df to walk me down the aisle, but that could only have happened if I'd waited until after my dm died.
Mostly it was my friends and some family who wanted us to have the parties afterwards - not us seeking our days of glory, or wanting presents. Our family and friends just wanted to be part of the general celebrations and to wish us all the best. And I had a surprise hen night when 4 friends wanted to show their support for me, when I was going though a difficult time.
I was also really glad that I hadn't eloped without telling people. Although we were sad that the circumstances meant we were not with all or family, I felt very supported by knowing that they were thinking about us at the time of our marriage, and toasting our health. We also had a very personal ceremony, and I printed copies for dm, dmil, and dsis, so they knew what we would be saying.
OP, I do really understand your disappointment, and upset. I am sure that my dmil felt similar, but the best thing that she and others did was to accept our (difficult) decisions and celebrate with us wholeheartedly.