I’m not Jussi but while waiting for his reply, see my thread which might help:
Using Deezer, Spotify, Apple Music, Qobuz, Tidal wtih HQ Player
Whether or not you need/want automatic sample rate switching is the key to which solution is best for you. As well type of digital connect your source/s have.
For example a CD player (like you mentioned in #3) is fixed at 44.1kHz sample rate and usually have TOSLink output, so you wouldn’t need RME ADI-2 for that.
You could use miniDSP USBStreamer B:
https://www.minidsp.com/products/usb-audio-interface/usbstreamer-box
The main advantage RME ADI-2 adds is some additional input types and auto sample rate switching.
As well as analogue input for the “Pro” model.
If you clearly define your needs with above, it will also help answer your question #2