If I understand your "||" meaning;

Yes,
Keyboard MIDI IN would go to Focusrite
The output of the mixer audio would also go into the Focusrite input (or just plug the mic into the Focusrite, which is likely better)

The Focusrite USB would then go to the computer.

Now think about the output side; the computer would likely also send the signal back to the Focusrite (including the added BiaB tracks) ..
This is part of what makes having the Focusrite mixer feature available so valuable. Being able to monitor the In/Out signals with some authority.

Essentially your existing analog mixer becomes a simple preamp/mixer for the Focusrite input.
Having another digital mixer (the Focusrite) further down the chain adds another layer, but also extends the flexibility of the system. The In/Out signals can then be balanced in the final result.

I do not work here, but the benefits are still awesome
Make your sound your own!