Stylus Studio is an award winning XML Editor, Generator and Development Environment.
(1) You can validate a sample EDIFACT submission in less than 5-seconds.
(2) The way in which the XML data is presented makes it very easy for a developer to visualize how individual items are nested within a single EDI file.
(3) There should be no problem handling custom elements, segments, etc. because the EDIFACT dialect files are totally customizable and extensible. And the process for adding a custom item is straight-forward.
(4) The software is supported by clear documentation, on-line videos, and lots and lots of examples.
(5) They pretty much have written the code you will need to integrate the software's functionality into your server's environment.
(6) Company has been responsive (a live person -- a developer, I believe -- has answered when I have called them).