Bit the bullet and updated firmware.
Works fine!
Even if it would've bricked you could've booted Hourglass9 or such to restore backup.
Btw I don't understand what you mean by two FBI's? When you inject FBI into H&S you then have FBI instead of H&S .. You then install FBI with the said app, and afterwards you're supposed to inject H&S back. Why didn't you just inject hs.app back? Note that it is possible that even after injecting hs.app back the icon of H&S remains as "FBI", but this is just a visual error - it's the H&S app and it works as intended.