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To follow up on this, we recently changed our infrastructure to support syntax merging and your forums are now being reasonably parsed.  This has been long requested by the semantic web community and this was a nice catalyst.  But it would still be better to switch your Comment -> mainEntity -> DiscussionForumPosting predicates to use "parentItem" instead of "mainEntity".  The next release of schema.org should have a change to make that valid in schema.  mainEntity is a very generic type that more often is used to refer to the actual semantic entity of the topic (like the car being talked about on a forum) rather than structural relationships so parentItem will work better.