You can disable the wallpaper in connection Properties -> Additional properties -> Remove wallpaper. As for a running screensaver, it gets switched off once you connect to the remote PC in the Full Control mode and do some action, like a mouse click.
There are couple of reasons why a new ID is generated:
1. You check "Generate new ID and send via email" option on step 3 of MSI configuration process. Don't do that if you don't want a new ID generated.
2. You leave "Keep remote Host's INternet ID settings" check box empty on Step 4. You should check that checkbox if you want to keep the ID of the Host you are upgrading.
3. Finally, on the last step when Host settings window appears at the end of the configuration process, do NOT generate the ID in the "Internet ID connection settings". Otherwise, if you do, it'll generate a fixed ID value that will be the same for each installation you use that output installer file for.
3. Didn't get the question. How can you possibly upgrade from Viewer to Host when these are completely different modules with different purpose? Did you mean Agent rather than Viewer?
4. This is what the Agent is supposed to be used for. The remote user can simply download it right onto their desktop and run when the remote session is about to start. The ID and password are then displayed in the Agent window.
As for the Host module, it was intended primary for unattended access when no user intervention is necessary for the remote session to start.