We had something similar. Our daughter's tooth was hallow in the middle - the dentist said "it was born that way".
She too had the gas. It helped because the procedure is a little long and sounds funny.
I would take the professional's advice. Try to play it up to your daughter. Explain that her tooth is sick. But it's no big deal because the dentist knows how to fix it. And then she can have lots of her favorite foods like Go-gurts, ice cream, jello, etc. We stocked up on these favorites to have them on hand. Although our daughter faired VERY well and ate regular food on the other side of her mouth that night.
You can ask the dentist if Advil or Tylenol BEFORE going in for the procedure will minimize swelling and pain without having any negative side effects.
Our daughter's tooth got drilled, cleaned and then a crown applied. It saved what was left of her tooth and more importantly protected the nerve and jaw from future infection and complications.
That was 2-3 years ago and we've had no problems since.
FYI - we were told NO taffy or super sticky candies like Gummie Bears. We are very careful about this particularly with birthday goodie bags and Halloween candy. They can adhere to the crown and pull it off or loose; that can lead to an infection.
I know it's scary to hear "root canal". I sincerely believe that we've come a long way in medicine and dentistry. I hope your daughter does well. Just be glad that you have a dentist who knew what he was looking at.
Put on your Mommy Brave Face and Smile. Tell her everything will go well and she'll be just fine.