A:Answer850 is just a newer version of the 840, but with better specs. Different button and camera location, so a 840 case won't fit a 850. I have both, I tried.
A:AnswerYes, Skype has instructions on how to install and perform a video call. Getting started with Skype (Android tablets) - https://support.skype.com/en/faq/FA12160/getting-started-with-skype-android-tablets and Making a video call (Android tablets) - https://support.skype.com/en/faq/FA12191/making-a-video-call-android-tablets. The Android operating system also has the Hangouts application (https://play.google.com/store/apps/details?id=com.google.android.talk) that can be used, if wanted.
A:AnswerI do not think that it will work, the webcams on the different units are in different locations on the front and back. I think that this is a pretty new product so the covers will come out soon, if you would like to wait.
A:AnswerFor more sound, especially when watching a movie, I connect my daughters tablet to her Bluetooth speaker. There's no other way except using headphones.