Hello Bryan!
Welcome to planDisney! Your family will have such a delicious time at
50's Prime Time Cafe in the next few weeks. It might just be my favorite restaurant at
Disney's Hollywood Studios!
While I do enjoy the banter with the Cast Members pretending to be my distant family members I understand how that may not be for everyone. When you arrive, make sure to tell your waiter that you are not interested in participating. Many times, these very discerning Cast Members can tell if a guest is uncomfortable and stop but I would not let it get that far and let them know you are not feeling up to much interaction today right at the beginning. I found that during my last visit, I had no negative interactions or admonishments but good lighthearted fun. I even joined the "clean plate club" and earned a sticker for my "hard" work! One of my proudest achievements.
As for side substitutions, you will find no issue in switching out your mashed potatoes to whatever sides they have available. Just ask! My husband loves green beans and made to request that for his side dish. Unlike you, mashed potatoes are my life force and will be my chosen side every time. (Fingerling potatoes can take a hike.) Your side dish alternatives may be different but I remember there being a vegetable option such as corn and peas along with macaroni and cheese, and French fries! Family meals can be a pleasant experience with all your favorite foods and no drama- at least in Walt Disney World!
I hope you have a delicious time! Don't forget to make room for dessert. We loved sharing
"Dad's Favorite Chocolate-Peanut Butter Layered Cake" last time and I highly recommend it!
Happy Planning!
-Nimmy