FMIL rant that took me by surprise

UPDATE: Thanks all you lovely people! It was so useful to get some perspective and most definitely helped me empathize with FMIL. We talked more in depth and while she is still not overly happy and is still being a little condescending, it is clear that it's something we will both work through. So, feathers were ruffled, but we didn't kill the bird. In the end I'm glad we told her, because I believe she would have been very hurt had we informed her after the fact. Thank you again weddit! And happy planning!

Edit to add a tl;dr.

So, I found out yesterday that a medical procedure I need has been denied, so the FH and I are getting legally married down at the courthouse tomorrow. That way I can get this procedure done before the wedding. I'm in a lot of pain and the doc wants to do this so I can enjoy the big day. The procedure is actually to kind of hold me over until a follow-up surgery which is obviously more invasive and requires more recovery time and will happen after the wedding (which is April 30th.)

So, the FH and I are treating this as just kind of filing paper work early. It doesn't feel like our wedding day. We're squeezing it in between work and class. We won't get dressed up, we won't count February 25th as our anniversary. We just need to do it ASAP so after the 30 days it'll take to get on his insurance I can get this procedure done ASAP.

FMIL wants to be there. I do not want her to. Selfishly because my mom won't be there (not only is she unable to take off work, but she totally understands that this is NOT emotionally significant and is only to end my unnecessary pain.) Also, If we wait and do it when they could both attend, then it'd feel wrong to not invite our fathers (one of whom is out of state and the other out of country on business til next week) and if we invite our various father figures, what about my grandmother who practically raised me like a second mom? Or his grandmother? I'm sure she'd feel slighted if she weren't included. And then it won't feel like just a technicality. It'll feel real. And all the planning, all the anticipation for April, my dress, his suit, all the money so carefully budgeted and selectively spent, all the amazing people we want to share the day with... It doesn't seem fair.

I have an excellent relationship with FMIL, so I explained to her myself that I would feel uncomfortable with her being there. I did not go into great detail, just that it needs to just be FH and I because it's simply a technicality, not our wedding. I didn't really have time to examine my own feelings on the matter before speaking to her. I didn't expect her to invite herself or to have to tell her no. She's accepted it, but it's obvious I've hurt her feelings. This is the ONLY thing we've ever had occasion to disagree on (a fact we are both seriously appreciative of.) She has, totally out of character, sent me some cryptic texts today alluding to her unhappiness with my decision, but has not out right confronted me. I genuinely feel terrible to have caused her pain. But I'm also a little resentful that she is making this choice to go to the courthouse harder for me and her son.

I'm so tired. What should I do? Am I making a mistake? What are your thoughts weddit?

tl;dr: FMIL wants to crash our rushed trip to the courthouse tomorrow even though no one else can go or is invited and it's just to get legalities out of the way so I can get a medical thing done so I won't be in pain for our actual wedding in April.