This isn’t an issue with abstract functions or anything like this. I think it might be related to the DLL symbols again: Microsoft’s nutty DLL-import scheme requires symbols that are imported from another DLL to be prefixed with _imp, and symbols that are local to this DLL are not meant to be prefixed.
This is why you see the _imp prefix on do_unlock_and_reload_ram_image@Texture in the output of nm. (You should get used to using dumpbin /exports instead of nm; I think it’s more reliable on Windows.)
It’s suspicious that the _imp prefix is absent when the error message is reported by link. Almost as if BUILDING_PANDA had been defined when compiling some parts of the framework directory. Make sure that BUILDING_xxx is only defined when compiling the code that is going to go into library xxx.
David