Is it possible to store guixt.ini and the user catalog (as well as the scripts) on the application server, or does one have to use a central Windows server?

Both are possible via the notation "SAWR:.." in SwitchTo / UserCatalog.

With SwitchTo, please observe that in the local guixt.ini
RFC user and password have to be given, e.g.

IgnoreRegistry Yes

StartHidden Yes

RfcUser guixtrfc

RfcPasswordEnc wavfofmfhfj