Changing the hidden mode: I know that the command "guixt.exe visible" displays the GuiXT screen when GuiXT is started in "hidden" mode. Is it possible to change the GuiXT starting mode "hidden" directly in the Windows registry?

Yes, this is possible. You can use the following .reg file:

Windows Registry Editor Version 5.00

[HKEY_CURRENT_USER\Software\SAP\SAPGUI Front\SAP Frontend Server\Customize]