Well, I'm sorry for stating it with such certainty then, but this comes as a big surprise to me. It seems obvious to me that they shouldn't be merged for the reasons I stated. I'm aware that you're the style leader, but this doesn't seem supported by the guidelines at all.
A release consisting of only a remix should clearly have this type.
To be fair, remixes aren't mentioned in What should not be grouped together? or Other situations either. It probably should be mentioned in one of the three.
This relationship can't be used if the release group is shared.
Respectfully, this sounds like a very strange thing to say about remixes. As I'm sure you're aware, many remixes are entirely different from their sources. I would definitely say that they're more different from the source than a remix with no changes but an additional verse from a featured artist.
Along with the release group type, the shared release group artist is another good reason not to share a release group, although you personally seem to be fine with keeping them split in that case. But I think keeping them split in some cases makes it a weird decision to merge them in other cases.