Alter results programmatically
Instead of a pre-render function, I think the correct spot to modify values is actually hook_views_post_execute().  This is invoked after the query has run, but before the data has been passed to rendering.


  Adding output to the view can be accomplished by placing text on $view->attachment_before and $view->attachment_after. Altering the content can be achieved by editing the items of $view->result.


It has been a while since I've personally played with Views Data Export so I'm not 100% sure why you're seeing only some values change and not others, but I recall the rendering of the export being a little abnormal compared to the usual render process.