I was able to resolve the AEM 6.0 related issue and will look into the other issues today and Monday.
Someone @ Adobe felt it's necessary to modify (enhance) the /libs/mcm/components/commands/newsletter/flighttest.POST.jsp component but I'm inclined to believe this requires a little more testing. This component produces a couple of errors and I finally decided to revert to the old AEM 5.6 version of the JSP and out of a sudden .... MCM email works in AEM 6.0. Please find attached a small sequence diagram. Hopefully other people will benefit from this information/knowledge.
regards,
Wim