https://community.tableau.com/thread/104502
Here's the fundamental technical summary: A twb is an xml document. A twbx is a "zipped" archive containing a twb + any external files associated with that workbook, such as extracts and background images.
Reader is defined as only opening twbx, so #4 is a function of "what Reader allows" rather than any twb/twbx difference. This is also the same regarding #2: technically, a twbx need not contain an extract (you could use this to zip your other external files up with your workbook, while still connecting to a server in Desktop) - it's just that Reader is not allowed to connect remotely.
