You were frustrated by the situation, so I can understand the long post.

Honestly I think you're going to notice a degradation in quality if you upsize the papers to 12"x12" at 300 dpi. As far as I know there is not magic trick, just resize them as normal.
Most of the freebies I've downloaded are correctly sized and a great addition to my stash. You can't go wrong if your favorite designer is giving something away free either.
If any of these were freebies from the downloads here, please let us know.