Yep, it works in download play.Yes, as long as both players have them installed. I´m not sure about downloadplay, but I think yes
Without using official tools, you can't do anything.I´m working on HD Peachs Gardens right now. Here are some screenshots:
The driver´s texture is made by Stewie1.0
But I have some problems: The background is sometimes in front of the map objects and makes them invisible and some textures (like in the last screenshots) appear invisible in a certain view... I have no idea how to solve this problems...