1, No, it can record 48 audio tracks and unlimited MIDI tracks.
2. Yes they offer an upgrade discount.
3. Yes you can use it as a controller. The Yamaha site has templates for cubase. Install it as a generic controller:
AW2816 MIDI Remote Template for Steinberg Cubase - Read Me
As far as directly importing audio files into Cubase...I'm not sure. They reference optional audio interface cards.
However, it worth remembering that Yamaha
OWNS Steinberg (Cubase) so if it's possible with anything is possible with Cubase. I don't think the LE4 version would have any limitation on that aspect.