In addition, when I make the psql.exe of this installation connect to the database via port-forward, then I cannot see that DLL being loaded using sysinternals process explorer.
That sounds very strange, as psql cannot operate without libpq. It's been a while since I did anything "in anger" on windows, but postgres certainly doesn't do anything to try and "hide" from a took like process explorer. Either you must be looking in the wrong place, or there's something else going on somewhere on the system that prevents it.
