(1) Please indent.
(2) Consider syntax highlighting. http://softwaremaniacs.org/soft/highlight/en/ will do it for you.
We wrote similar apps for Outlook 2013, but didn't see much traction. Also, compared to classic COM plugins (which are also still supported by 2013), the API is very limited. That combined with no backwards compatibility makes me think that it would have been better to implement this as a COM plugin for better coverage and functionality...
Nowadays XHR is a bit limited as far as exception handling and other niceties you'd like for API calling. But Excel VBA can access anything through COM interop, such as a more robust C# library using .NET objects like HttpWebRequest.
Excel also has a notion of web queries, to populate a range by making an HTTP request and scraping a table out of the HTML. I think that existed as long ago as Excel 2000 and was brought out to the UI in 2007. It doesn't play well with websites requiring a login or a nontrivial click path, though. I have vague memories of wrestling with this, through the spectacular method of launching an IE window from Excel and relying on the MSHTML layer to share the login cookies between that and Excel!
I can't speak to Excel integration, but I think people are doing that.
If the user experience is appropriate, then apps typically work very well in both Word and Excel without any changes to the application code.
