As a mom of someone that has IC I'm not sure that a pessary is the best thing to for you.
I mean she is incredibly sensitive to just about everything and anything touching her in the nethers. I am wondering why your Dr would encourage this over a patch, and just plain old lube when needed.
I also see how if it is coming out frequently that you could develop yeast infections. It is the perfect habitat for it to grow, warm, moist, dark, and the moisture is being put back in w/ the estrogen it is giving you.
Couple of thoughts.... I would get on a probiotic, a quality one like Align, or Florastor, and take it twice a day, and add yogurt into your diet at least one time a day.
I would also treat this yeast with cream and diflucan- call the GYN and ask for it it's a one and done dose of medication to cure the yeast as well.
Pessary's can be great especially if you leak at all, or have any sort of prolapse, they even make disposable ones you can get in the feminine hygiene aisle at the store. As someone who is post-the pause, and has a tiny bit of leakage with coughs and sneezes, I have considered trying it to help when my allergies are bad.
Anyway time for another discussion w/ your Dr.