Regarding crossfading between articulations:
You cannot - AFAIK - crossfade between cells, but you can load both articulations in the same cell, but in different slots (i.e. sus in 1a and trem in 1b). Then you need to assign a free CC to slot-xfade and can automate the crossfade seamlessly by increasing/decreasing the CC value. You just need to make sure that you have that white circle between the slots 1a and 1b checked, so that an "X" symbol appears inside. If you leave that unchecked, both articulations will be audible simultaneously instead of crossfading from one into the other.
I'm afraid I don't really understand the first question, I apologize. Since you are able to trigger multiple matrices via KS, isn't that already what you're trying to do? The first matrix would be the sustains matrix, the second one would be the short note matrix etxc. Hitting the keyswitch would automatically force the Vienna Instrument instance into the corresponding matrix, which also automatically applies the defined articulation triggering method within (velocity, speed etc.) I suppose there's something I'm misunderstanding?