If you want to use your computer as an arranger you need arranger software, not Cubase or other sequencing software.
http://www.varranger.fr/
http://www.live-styler.de/home/
http://www.livearranger.com/

Then you can set up your master keyboard to operate like an arranger.

Not sure why you can�t get the full dynamics on your master keyboard and piano samples, as I have found the default settings to work pretty much spot on. (You must have changed some setting without realising it)

If your using your computer for composing, set it up for music and not anything else (Set up different user accounts and use fast switching if you want use other things, just remember not to do it at the same time)

If you want full integration with everything both arranger software and computer software have to offer, then go for an open keyboard that is specifically designed for the job, Such as;
http://www.lionstracs.com/store/groove-x7-red-p-251.html
http://www.youtube.com/watch?v=8hAwYfXRv0c&feature=player_embedded

Hope this helps

Bill