I think an e-collar would definitely stress her out more. The kitten definitely could have been a stressor. How long has the kitten been gone for?
Is she alone? How long have you had her? Gliders should always be kept as a pair. She could easily be overgrooming from loneliness, as well.
Keep an eye on her for any open wounds because then she will definitely need further help, but for now, you just need to figure out the cause of her stress and fix it.
The kitten leaving could very well have solved it already! The hair should grow back in a few weeks. If it isn't back in a month, then she's still overgrooming. But it does take awhile and it can be hard to see such a small change, so don't worry if you don't think there's an improvement right away as long as it doesn't get worse!
I love the purple wheel by the way! :)