I'm sure others here can be more help than I can. I tried integrating a DS into my rig, and gave up.
I now have a FA; Roland appears to implement midi control the same on both, and unlike Yamaha, Korg, and the Integra 7. For layers on the original midi channel in multi-timbral mode ('studio set' on FA, I forget what it's called on a DS), only note info is accepted. No mod, bend, sustain were accepted coming from external MIDI. However, every other channel worked with control information. (Note I only tested these items bc that's all I use onstage).
The only thing that worked as expected was turning on 'Remote Keyboard', which may be a FA-only thing. But it caused many other issues making it useless as an option. When I was testing out a DS, the Matrix Control did nothing useful for my setup and had no impact on the first layer.
It's a mystery to me why Roland did it like this on their KBs, especially when the Integra 7 functions as expected. I might get rid of my FA.
Hope this helps!