1. You cannot stop it as BB will generate the marquee as it is coded in the background logic to show a marquee even in themes that do not have a GameMarqueeView.xaml. So the only way I have found is to hide it by adding a black background. Placing it above Panel.ZIndex="0" will get it to cover the marquee. Adjust the column and row span to your grid.
<!-- BLACK BACKGROUND -->
<Border x:Name="Blackscreen" Grid.RowSpan="8" Grid.ColumnSpan="8" Panel.ZIndex="1" Background="Black" Opacity="1" SnapsToDevicePixels="True" RenderOptions.EdgeMode="Aliased" ></Border>
2. To make the background show platform specific use a multi-binding. Something like below just direct to your folder path.
<!-- DEFAULT MARQUEE -->
<TextBlock x:Name="MarqueeBackgroundPlatform" Visibility="Collapsed">
<TextBlock.Text>
<MultiBinding StringFormat="LAUNCHBOX_ROOT_FOLDER/Images/Marquees/Platforms/{0}.png">
<Binding Path="SelectedGame.Platform" />
</MultiBinding>
</TextBlock.Text>
</TextBlock>
<Image Source="{Binding Text, ElementName=MarqueeBackgroundPlatform, FallbackValue='LAUNCHBOX_ROOT_FOLDER/Images/Marquees/Platforms/Default.png'}" Grid.Row="0" Grid.RowSpan="4" Grid.Column="0" Grid.ColumnSpan="8" Panel.ZIndex="3" Stretch="Fill" RenderOptions.BitmapScalingMode="HighQuality" />
3. I am not sure of. I know for theme's some of the views (such as WallGames) you can create a dedicate view xaml per platform. So it might be a similar process. I am not sure if that would be doable with Marquee Views as these views have not received any updates. I will check on it.