You were wrong to do the "If you don't do X by 2:30 you don't go." Your daughter did not really believe you would cancel anything as hard to arrange as this trip. I think you already know that you used the "she didn't do X" as an excuse when you had a real and legitimate reason to deny the trip that you did NOT use -- your child's health.
You were right to be very concerned about your child's health. A concussion? Seriously? These days, a concussion often means days and even weeks of a kid staying still, not even watching TV or reading -- three friends' kids have had concussions this past year and in every case, there is NO way that those kids would have been allowed to fly and be tourists just eight weeks after the concussion. The doctors would have probably gone ballistic if the parents allowed it.
And celiac? Yes, folks in Europe do live with it as someone posted -- but I've traveled there repeatedly and eating in restaurants would indeed be a real issue for your child. Period. Daily. Grandma would have to commit to going to stores daily to get other food. And does Grandma know how to read labels in French or German to know if a food has gluten in it?... I have a friend who has celiac and it's something to take seriously; your child could end up the hospital overseas, and it sounds like Grandma does not take the disease seriously. Your daughter is nowhere near ready to advocate for herself or stand up to Grandma when needed.(And your daughter does need to get to know her health needs and be more assertive, but she has time to learn that before college -- work on it.)
I'm surprised some posters are saying here that you should have sent your child on this trip -- I think they missed the concussion and the celiac flare-up and the adverse reaction to medication entirely and are focused just on your using the "do this by this time" excuse to deny the trip. And yes, it WAS a clumsy and obvious attempt by you to find some way, any way, to back out of this trip; you should have used the very real and pressing reason: My child had a concussion and her disease has flared up, and she should not travel at this time. You missed the boat there and end up looking petty when you had a sincere and real reason not to permit this trip.
Of course the real issue is that you let Grandma plan it at all. Standing up to her years ago on the promised trip would have averted this mess, but again, you know that.
Where is your husband in all this? Is he also angry at you? Is he siding with you or with mom or not taking sides or just doing the "Leave me out of it" routine that some people do? She is his mom; she lacks all boundaries; HE needs to deal with her from now on forever. You lack the ability to be direct with her, because hinting around for years that "we haven't said yes" is weak, frankly -- can you see that some people just read that as "Yeah, but you haven't said no so I'm going ahead here!"
It is VERY telling that your daughter finally admitted she is not always comfortable around grandma. That is your red flag and you need to heed it, and to talk to your husband about it. This situation sounds like one in a family I know where the parents ended up having to keep an older female relative away from the teen daughter because the relative was contacting the girl behind the parents' back, turning up at places the girl was without her parents, etc., and it made the girl and parents very uncomfortable. Grandma is very invasive. You were wrong to cancel the way you did, and as late as you did, absolutely. But she spent years talking to your kid behind your back. That's got to stop and I hope your husband sees that too, or he may end up pushing your child to have contact with Grandma that your child does not really want.