No! Use port 9951. SL opens its own OSC listening port which Zynthian talks to. You can communicate directly with SL via its OSC port 9951. In theory any clients, including Zynthian should be updated, i.e. a change made from your remote should update Zynthian display.
Yes! Mixer is fully controllable via OSC. @ronsum and I created an Android client for it some time ago but that used an app that is no longer working or supported. Mixer OSC is via port 1370 with paths:
| OSC Path | Value range |
|---|---|
| /mixer/faderX | 0 … 1.0 |
| /mixer/balanceX | -1.0 … 1.0 |
| /mixer/muteX | 0.0, 1.0 |
| /mixer/soloX | 0.0, 1.0 |
where `X` is the fader strip index. 255 for main mix bus strip.