The only property you need to use is IAGREE=Yes. No extra shortcuts or toolbar will be installed. The other properties only work in the UI sequence. I prefer to build a transform in which I put the property, the preventautoupdate registry key and remove the autorun registry key.
Following up on artini's note, you can extract the Messenger MSI's from the download executable using /C /T:<path> command line parameters.
Another way to disable autorun is to remove the registry entry 'HkcuAutoLaunch' from the registry table.
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