I'd support the Hatch + Kay piggyback 5th starter idea and bolster the BP with Thornton as a setup guy.
Thornton isn't a bad 5th starter, I'm just not sure how to feel about his stuff vs lineups 3 times per game. His stuff seems to really diminish by the second time through the order and really much so by the 3rd time. He also has a herky-jerky delivery with a tendency to tip pitches and get exposed in long outings. While the high spin-rate stuff and uptick in velo has potential to be lights-out in the BP.