ADAT will output DSPs outputs (8) and/or AES-EBU inputs/ADAT inputs/FW inputs (no DSPs in these last 3 options).
You can create the described routing and output all thru ADAT at 48KHz; at 96 you'll get 4 channels only. FW is not recommended at the moment for 96. AES is fine.
Consider that 96KHz is using A LOT of resources and you may not get all the processing power for your 4 synths. Better staying at 48.
You can't disable where the signal is tapped from (analog from AES). It's a structural aspect. Nevertheless I wouldn't be worried at all about this. The routing options are many. You'll see when you'll get the unit.
The last routing is possible too, 2dsps in parallel each one fed by analog or AES.
Yes routing can be switched via MIDI. A studio only recommended operation!