I am a vet tech and at our hospital we have the same forms stating that you understand that there can be risks asscociated with anesthetic and also if your pet was to soil itself, become injured from an escape attempt or refuse food while at the hospital that we will do everything under our control to prevent but accidents do occur. It does not mean you are signing your cat's life away just simply that you understand that these things can happen.
On the other hand you are not helping your cat any by NOT having the dental work done, a bad case of gingivitis can lead to many problems such as kidney failure and heart disease. I would just make sure that you atleast have a small pre-anesthetic panel of bloodwork run to check out all the basic organs to make sure they are working properly and if your vet allows it and would make you feel better-ask and see if they will let you watch while they are cleaning the teeth-it is interesting to learn about and you can be there if (heaven forbid) anything should go worng. Make sure they hook your cat up to IV fluids, that will help keep the anesthetic to wear off easier.
I do understand that it is a scary thing to sign those forms, but remember when we have any type of surgery or anesthetic we have to sign the same forms. Good luck with your kitty and please consider having his/her teeth cleaned. Your kitty will be better off for having it done.