.  
4.  It will be interesting to see if the SD90 can be controlled using the buttons and sliders 
on the A-800 Pro keyboard.  (I don't know how to make this happen yet)
I took a quick look at the SD90 manual.  Only a few pages. It is more like a brochure rather than a manual.  I suppose you can refer to the SD9 Tutorial that I think AJ wrote as he had done for the SD40. 
Anyway, it seems that you won't be able to use the buttons/knobs on the A-800 to control the SD9.  I don't think the A-800 Pro sends sysEx the SD9 needs.   I understand the Akai MPK series 2 can send sysEx.   Some legacy Novation controller keyboards and control surfaces can program and send sysEx.   I use some midi foot controller surfaces to send sysEx.  One of them is the Gordius Big Little Giant 2.