In Kodi, you need to go to settings, system, audio output, and enable passthrough, choice the audio output and click the box that apply to what your receiver is capable of decoding. Then it should switch automatically when it detects a signal that is DTS/DTS-HD/DTS-MA/DD/DD+/DD+EX/etc. Mine can do just about everything I think.
Depends what you are playing the files on? Play them on MPC-HC or Kodi which both allow passthrough to be enabled, then it will simply pass the audio, untouched, to the receiver and let the receiver deal with it. Something like Windows Media Player that doesn't have the DTS/DD codecs available, can't decode them properly and falls back to straight multi-channel.
I run Kodi as my HTPC and always enable passthrough to let my Denon do all the decoding. Denon paid money for the license's to be able to decode them, so I let it. DTS-MA sounds awesome on my 6.1 through my Denon.
Yes. If I play a DD or dts source through kodi with pass through on my receiver will change to that mide and display it on the screen. For kodi pass through switch it to the wsapi driver enable pass through and click all the boxes.
Should be fine. Also what gpu do you have connect to the receiver.