Yup, I use dRambo as my main sequencer for both software and hardware.
Yup, you can use the Digitakt as an audio interface if I remember correctly though I don’t have a Digitakt to test this myself.
As long as the audio interface is class compliant then it will work though you can only use one audio interface at any one time, a limitation of idevices and in regards to Program Control messages?
Yes, dRambo can send PC messages, have a look in the Midi section but you cannot send Bank Control message yet.

I know some people use drambo this way so it’s definitely possible.