First, I would never advise that you work with backups. You work with your originals.
Now, if you are in the unfortunate situation where something bad has happened to your originals, I would make copies of the backup catalog and photo files using your operating system, so that you now have a new "original copy", and move them to the location where you keep your originals. I would still leave the backup alone.
Then, open the "original" catalog as normal, make your changes — and remember to make a new backup (as you described), as the previous backup is now out of date, and then close the catalog.
Which brings up a broader point — there may be more efficient ways to operate, and which would save you the time and effort of making backups. In my mind, the most foolproof way to implement a backup strategy (which therefore is the safest way to implement a backup strategy) is to automate everything. If you rely on your own good will to do this, the chances that something won't be backed up are significant. If you automate everything (and there are plenty of software available to do this), the chances that something won't be backed up are close to zero. Humans are more fallible than computers at these tasks.
