Benadryl will not help with congestion and may make it worse. It'll dry you out. That's what anticholinergics do. Why do you thing one of the side effects of taking too much Mestinon - which does the opposite as Benadryl - is excess saliva?
In home remedies that won't hurt you are steam, hydration and rest.
You need to report that reaction to your neurologist right away. And if you are having that much trouble getting over the flu, you absolutely need to see an internist immediately. You might need more supportive things like oxygen or other meds. But only THEY should be advising you on any drugs, even over the counter ones!!!
Please, everyone, no more experimenting with drugs and supplements. They are not benign when you have MG. They can put you into an MG crisis!
I hope you will feel better soon but you probably need more care . . . from a DOCTOR!

Annie