TLDR/Question
How can I best assign unique IDs to (ideally all) of the elements in the XML that describes a Word document such that I can read/write those unique IDs from a Word (2013) Add-In?
Additionally, solutions describing ways I can get a good diff of two Word documents might be helpful but this is not the primary question.
Background
I'm creating an application-level add-in for Word (2013) using VSTO. Part of my task involves diffing an original Word document W with a modified W' so that I can then process the diff for another task. While Word clearly has the capability for diffs/merges (available in the "Review" panel in Word 2013) thus far I have not been able to find a way to programatically extract the diffs.
Therefore, I plan to get the XML for the documents (e.g. using Range.WordOpenXML) and diff them. There are a number of published algorithms for diffing XML documents (i.e. Diff(W.XML, W'.XML)) where the accuracy of the diff is largely dependent on being able to properly match the XML elements from the two documents.
Proposed Solution and Its Problems
Therefore, I'd like to be able to assign a unique ID for every element in the XML of the Word document that I can access from my Add-In. In this case a solution would be something like importing a custom namespace into the package called mynamespace and adding the attribute mynamespace:ID=*** for every element in the DOCX package. The attribute would then be accessible via Range.WordOpenXML.
However, simply using mce:Ignorable, mce:ProcessContent, and mce:PreserveAttributes as detailed at http://openxmldeveloper.org/blog/b/openxmldeveloper/archive/2012/09/21/markup-compatibility-and-extensibility.aspx does not work. The modified Word document loads without any issues, however I cannot seem to find any of the attributes, additionally saving the document removes all of the added markup.
From http://openxmldeveloper.org/discussions/formats/f/13/p/8078/163573.aspx it appears that this process of using custom xml via the Markup Compatibility and Extensibility (MCE) portion of the Office Open XML standard has become complicated over the years (patent issues, etc.). Therefore I'm guessing that my issues arise because Word's XML processor just removes all of the markup that it cannot natively process (maybe there is a way to hook into Word's XML processor and give it custom commands?).
For the future viewers:
1) There is absolutely no way to set any kind of id for most of elements, which can survive in Word (you can use any custom tags or attributes, but after MS Word opens the document, it's gone)
2) Only two elements can be used as id - ContentControl, they have ids, and bookmark (it is possible to make a hidden bookmark adding underscore before it's name, it works only from code), their name can be an id.
3) If tracking changes is enabled in Word, it is absolutely possible to see the diffs in XML, using Range.WordOpenXML and getting actual OpenXml from it, as explained here, for example.