I am just another WA users, but I can answer a few of these questions for you. :)
The first error you list has been shown in this section (and the Data Bridge section) about a million times, you could have easily found the answer to that one had you done a bit of reading. I'll answer it again anyway.
It is cause by eCart conflicting with the new extensions, but was always an irrelevant error. It can now be fixed by downloading the new version of eCart. Remove the old eCart from the extension manager and remove Design Extender (even if it is current!!). Install the new eCart and then the current version of Design Extender in that order and that error will go away.
Your second error has also been written about and could have been found had you done some reading, however it simply shows that Dreamweaver is trying to display a panel that no longer exists but is within your working DW layout. To fix it simply choose 'Window > Workspace Layout > Reset "-WORKSPACE NAME-" ' and it will also be resolved.
As far as the $149 for the new extension grouping.. that has also been answered about half a dozen times at least, so I'll let you find the answer to that or maybe some from WA will answer you here.