Personally I would keep her at the vet's until after the race.
My reason is this- when my cat Pearl was literally on the verge of death from kidney failure, we had her for several days at the vet's on a constant IV fluid drip. Unfortunately, we had to take a Fri-sunday trip- a commitment that we literally could not get out of.
The vet warned me before I left for the weekend that things looked grave, and that the constant fluids were her only chance, along with the antibiotics for her infections that were induced by the kidney failure. She was literally drooling bloody mucus, that's how bad she was. It was breaking my heart.

But though she was lethargic she didn't seem in awful pain, she would purr whenever petted, and I knew her only chance might be the vet's IV water hookup. So with heavy heavy heart I left her at the vet's while we did our trip. I knew she was in the best of hands and they were so gentle and loving to her there.
I couldn't bring myself to phone during the weekend. When I came home, I drove directly to the vets without calling because I was sure she would be 'gone' and for some reason I couldn't bear to find that out over the phone.

I got there to the waiting room, they said "just a minute"....then opened the door and Pearl literally came bounding out to greet me happily.
After that I had to give her sub-cutaneous water injections every other day for a year. She has been very happy and 'mostly' healthy since then, as long as I give her very wet food 'soup'.

I only tell you this not to falsely raise your hopes (because Pearl could just as easily have not made it, it was that close, and your Maggie is certainly in serious condition as well) but simply to say it's OK to keep her at the vet's on direct IV for an extra day or two while you go to your husband's race. It might be the best choice for Maggie, might give her that one little extra boost she might need from the extra fluid intake. Consider giving her that chance- her latest tests don't seem great but they 'have' improved a little while on IV, right? I would say if they start getting worse than before then there's no point in continuing.

I'll be thinking good thoughts for you.