I have no idea if this has to do with any changes that have been made to the site in trying to figure out the other glitches, but it actually let me respond to reviews for Song for the Undead under my own RokofAges75 account today. Normally I would have to log out and log back in as Rose's and my joint account, Double Rainbow. I had been responding to reviews for my other story and was still logged in when I checked the new reviews for Undead, and lo and behold, the respond link was there and worked. It hasn't worked like that for any of my collabs in years, so that was a nice surprise!