Messages -incoming & author.externid property questions

I'm using SuiteTalk to grab messages on a support case and import them into another system.  The author.externalID seems to always be null for each mesage - so I'm doing another search to find the author's externalID.

So, two questions...

Is there a reason the author's externalID would be null?

And, why is the incoming property always true?  In a standard support case form - I thought messages typed into the left text area would be considered to be from the case contact and thus incoming and messages typed into the right would be from the assigned to and thus not incoming.