Ok, just had a quick look at the qmake sources.
It expects the user to provide the make spec either as a qmake option( -spec or -platform ) or in the QMAKESPEC environment variable. If it cannot find it this way, it takes the default, which is located in QTDIR/mkspecs/default. The problem is that in the opensource version this folder does not exist. In the version precompiled for mingw it exists and it is the same as win32-g++ and for the commercial versions is set according to the msvc version of the distribution.
If it cannot find it in the default directory error it displays an error message:
Qt Code: 
  1. if(cmd & ReadConf) {             // parse mkspec
  2.             QString qmakespec = fixEnvVariables(Option::mkfile::qmakespec);
  3.             QStringList mkspec_roots = qmake_mkspec_paths();
  4.             debug_msg(2, "Looking for mkspec %s in (%s)", qmakespec.toLatin1().constData(),
  5.                       mkspec_roots.join("::").toLatin1().constData());
  6.             if(qmakespec.isEmpty()) {
  7.                 for(QStringList::ConstIterator it = mkspec_roots.begin(); it != mkspec_roots.end(); ++it) {
  8.                     QString mkspec = (*it) + QDir::separator() + "default";
  9.                     QFileInfo default_info(mkspec);
  10.                     if(default_info.exists() && default_info.isDir()) {
  11.                         qmakespec = mkspec;
  12.                         break;
  13.                     }
  14.                 }
  15.                 if(qmakespec.isEmpty()) {
  16.                     fprintf(stderr, "QMAKESPEC has not been set, so configuration cannot be deduced.\n");
  17.                     return false;
  18.                 }
  19.                 Option::mkfile::qmakespec = qmakespec;
  20.             }
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So I think it is ok and enough to provide a dropdown or a list with all the makespecs available on windows and let the user choose from them

...and also there is a problem with switches changes between different versions
No problem whatsoever. VS also has the option to switch between different Qt installations, but it does it internally.
It is just a matter of setting the environment variables to the desired installation.