1) Currently there is not an option to install the legacy plugins from command line. As workaround, to add this option you only need to copy the file "ForestPackPro2.dlo" to the 3dsmax plugins dir after installation. We will update our installers to add this option in next versions.
2) This message box comes from the 341 uninstaller, that didn't support automation parameters. Unfortunately there is not a workaround for this problem because the limitation is hardcoded in the old uninstaller. This problem is solved after updating to a newer version.