This turns out to have been a combination of a mistake on my part, and a non-helpful error message. One of the classes was missing a member of it's inheritance chain.
Personally I would have expected this to result in a compiler error ("Extending Undefined Base Class" or something similar), but instead I got linker errors for the virtual functions in said base class (despite that class not being included). But because no functions were actually referencing them, the linker could not provide a "referenced in" section for the error message. I still don't entirely understand why this set of problems with the code gave that set of error messages, but I was able to fix it.
Thank you Alan for your input, it got me looking in the correct direction. I would upvote you, but having joined immediately before posting the question, I lack the reputation score to do so.