After spending again some hours - and looking for the failure I'm making - I just did a quick and very simple API function call logger which allows to analyse in a spreadsheet whether functions are being called or not and how frequently they are called.
It's really easy so don't expect too much - but was quite annoying typing ;-)
Did spend some time to tweak the compiler/linker settings to have an Visual Studio project which really compiles and links Win32 and x64 versions in debug and release mode. There are still two warnings but no issue at all.