Re: Windows Store? Sideloading?
"Ever tried to have two Office 365 documents open, then change the back ground colour in one of the instances?"
Yes that works just fine - just tested. It's exact same version of Office 2016 installed as non Office 365 by the way. It's just different licensing.
"loading a document created in an older MS Office version into the latest version can completely screw up the formatting"
Only if you choose to convert to a newer file format. Office can quite happily load and save older formats.
"Then the constant re-arranging of the menu items"
They have not changed. Been the same since RTM.