I have the exact same sensation.. and it can be awful!!! At times it is so bad I sound like a frog when I talk!
Several months ago, i got a cough/cold which threw me into a bad bad exacerbation, along with barely being able to put one foot in front of the other, this throat sensation was terrible.. i felt like the back of my throat was touching the "front" of it.. When I opened my mouth to look, my soft soft palate did elevate like it should, so it was strange!! I could barely swallow - I was choking on liquids and solids, so stupidly just didn't eat/drink much for a few days till my IVIG (stupid I know)! I even tried to talk a few times, but no sound would come out, as it felt like my throat was closed.. after that next IVIG it was almost back to "normal", and just waxes and wanes since then

Have you had a sleep study? If you feel like your gagging/choking etc at night, they can do a sleep study and possibly put you on bipap/cpap or just add additional meds/ivig etc to help you a little more? (fyi- I don't have a firm diagnosis .. but they have seronegative MG high on the list)