I don't think you need to be feeling guilty at all...the reason for the lock is for her own safety. And you sleep right next door so you can still hear her and tend to her as needed, it's not like you're locking her in there and throwing away the key. It sounds like it's a practical solution given the logistics of your house layout. Since it's a chain lock and presumably she can still open the door a smidge, the bell idea is a good added extra so at least you know when she's trying to get out.