Is it possible to selectively erase time-machine backups for a given period?
You can open Finder and select your backup disk. Find the backups in the time range and delete them (shown in the image below). This will delete those backups on disk. This will properly delete the backups - if you inspect system logs you will find it handling these as requests to delete a backup, rather than as requests to delete a folder.
I would recommend doing it one at a time to avoid overwhelming the backup disk.
Deleting a backup will not delete files that were added in that backup from other backups. Each backup can be viewed independently, as a snapshot of your disk at that time.