Slime is the only one you can't defeat in grapple. Her entire body is a mass that act like muscle. You can't overpower this thing physically.
The mechanic you want, making the enemy back off after some time they grapple unsuccessfuly with you, already exists in the form of stamina. They won't be able to resist when they get out of breath and they will be forced skipping turns instead.
And choosing Continue in checks while skipping is also intended. Don't use the skip button if you read the dialogue for the first time and you want to choose options. Read them properly. The automatic choice on checks is very useful when you want to skip the encounter which you already read through, successful checks are usually the fastest.
