Okay meowmie, deep breaths now. I totally understand you being worried about your little baby. We all go through this, even though we know everything will be just fine.

To answer some of your questions. Yes, it would be a very good idea to keep her very quiet and away from activities for the time being. Usually your vet will tell you not to let her jump up or down from anywhere. In regards to the E-collar, usually kittens adjust well to it, mind you though there are some that don't. I pray that yours does as this will help to keep her away from the stitches. I would speak to your vet about what to do about keeping her from getting at her stitches if she doesn't do well with the E-collar.
All in all, if you can keep her secluded in a closed room, it would be best.

Prayers on the way that your baby girl will do just fine and ((((((HUGS)))))) for the nervous meowmie.