I've been using the CC for quite some time now...
1) You can stream to any screen that the CC is connected to.
2) From a PC you have two options, browser mirroring or from PLEX
3) It certainly does 5.1 but no DTS
4) Screen mirroring works great with no sync issues. If you have issues its your WIFI strength or router configs.
5) I see no appreciable reduction in quality when streaming 1080P MKV's
6) Using AllCast you can stream pretty much anything you can play on your phone to the chromecast including files on the phone itself
Its truely an amazing device for only $40 however if you are just getting ready to purchase now, i suggest getting the Google TV box instead. Its $100 but it allows native applications to be played right off the device. Think roku+chromecast in one package.