As for DMC, look in the “dm” subfolder of the QSpice install folder. You should see a file called VERSION (no extension). It’s a text file and contains the DMC version number – “857” on my system.
FWIW, as best I can tell, the last supported release of DMC is about 20yrs old. I spent a while trying to find out what version of C++ DMC supports; it appears to be C++98.
If you want to use newer C++ compilers for C-Block component development, you can. QSpice doesn’t know/care what compiler produced the DLL as long as it is a proper Win32 DLL and exports required entry points.