So... turns out, yea, I was doing something very stupid. Having read back the entire thread again, I've found you use
https://github.com/sim1/tf2itemsinfo
I've set up a compiling folder, put that in with the other stuff that's listed on Page 1, and now it compiles (despite other related things still having errors) and works perfectly fine.
I won't delete my post as I have a feeling there may be geniuses out there like me who run into the same self-made problems.
Anyway, thank you again for providing that version.
TL;DR: Use
https://github.com/sim1/tf2itemsinfo when compiling (and probably use that for your server as well, don't mix/match solutions like I did...)