In fact, I don't think the OLE API has changed in 15 years - perhaps 1 new method for running JavaScript. It is JavaScript that has all of the new functionality, but word searching and highlighting is at least as complicated and uncertain as it ever was.
One issue is that word boundaries are rather uncertain. Is / punctuation? This may change between releases.
Looking over your code superficially, it looks as if you are switching from word offsets to character offsets in your highlight list. I don't see how that could ever give correct results (except perhaps where the first word is a single character and that's what you are searching for) but I may be missing something.