__declspec(dllexport) is a MSVC-only feature. Hence, when compiling with gcc (even using winegcc) it won't actually export the function. As has been stated before, you need to use a .spec or .def file to export it.
Yes.
A DLL compiled as a winelib DLL won't be parsed by a tool that reads PE DLLs, because winelib DLLs are Elf shared objects. Use winedump instead.
Precisely. Easy to distinguish between wine and doze Dlls.
Use winedump instead.
I did. It dumps the CPlApplet() function nicely from all cpls (including one I made in Delphi).