Oh dear, it DOES sound like she has MG. First off, let me say how SORRY I am to hear about it. It can be a terrible disease.
However, it isn't ALWAYS terrible and can be made better or even go away for periods of time with the proper treatment and a little good luck.
I'll tell you about myself. I'm a 44 yo who has had MG since 9/26/98. I woke up that morning and out of the blue, couldn't swallow cheerios. Soon double vision followed, along with speech problems, chewing problems, neck weakness and to a lesser degree, limb weakness. I finally was diagnosed in 99 following several attempts at finding a neuro and working around my own denial of my symptoms (I'm a healthcare professional so knew what was wrong but I was also human and dealt with it likewise.)
I am seronegative, MusK negative, meaning I don't have antibodies in my blood. I've had a thymectomy (did the minimally invasive one) without success (it's not very effective in seroneg folks). I've tried Mestinon, the first line symptom controlling drug, without success as I'm allergic to it. I have used Prednisone (works like a charm but not without a cost to my body), IVIG (marginally effective but helps keep me otherwise healthy from colds/infections), and Cellcept (also marginally effective but with costs to my body).
I have not had really good symptom control. After a really bad first few years (which is typical of MG) I suddenly and without warning went into a drug dependent remission. I was symptom free for 2.5 glorious years. I even went off all my meds for a while with no problems! It was wonderful. However, 3 years ago, it all came crashing down and I've been struggling to find a manageable level of function since.
I no longer work much in my profession as I can't speak, see to drive well, etc. I now work from home and own my own internet business. I use my past knowledge from my job to help manage my symptoms. I use my blender to make smoothies, creamed soups, etc to help me get some nutrition into me. I prop up the head of my bed when I have swallowing problems at night. I simply don't talk much when I'm bad as it sounds so bad, people think I must be drunk, drugged or head injured (it's REALLY bad, in my case.) I cover one eye at home so I can function with the double vision. I've learned to adapt and enjoy the good times when they come. I'm currently on my second week of relative symptom free time and I am busy talking on the phone, eating all sorts of foods, etc.
Another thing I've noticed is that my symptoms get MUCH MUCH WORSE when I have hormonal shifts such as at ovulation time and start of menses. This is common with females with MG. If your dd has this, you may consider using some oral contraceptives as that does seem to help some women (but not me, of course! Sigh)
Your daughter will likely not have as hard of a time as I have. I'm a difficult to manage patient, and I'm one of the more rare ones. Mestinon will likely make a large difference to her as it will help control the symptoms for several hours at a time, and will make her more functional, able to sleep safely, etc. Once she's diagnosed, you will be given treatment options. Do your research and feel free to ask me anything. If you're not happy with your doctor, find a new one (it took me 4 tries to find the one I like.) I go to a university run neuromuscular clinic so I'm in on the cutting edge of research. The dept is big and somewhat impersonal, and I like it that way. Others are more comfortable in an intimate setting where there are more "warm and fuzzy" personal touches. You do what you and your daughter need.
And yes, go ahead and cry. Be there for your daughter. MG is something that affects the whole family, friends, etc. I found my friends scattered like roaches when the light is flipped on but I now know who my TRUE friends are, and I've made others in the last few years who know about and accept my problems.
Enjoy the good times! Since having MG I've been to Disney World and did everything, including the worlds tallest waterslide, been to Cancun and swam in the surf, hiked mountains in Colorado and have planned a trip to NYC in November. I choose to live my life despite the MG and with some careful planning, it usually works out well.
I hope I haven't scared you, and I want to stress again that I'm an unusually difficult case. 90% of folks have a much easier time of it than I have had. That being said, if your dd suddenly has trouble even managing her saliva during the day, or cannot eat food or drink for 24 hours, go to an ER. That is serious and she needs to have support nearby. Tell them she is currently in diagnostic process for MG.
I wish you well. Please keep me posted.