Osuna's innings per year
2011: 19.2 (MEX)
2012: 43.2
2013: 42.1
2014: 35.1 (minors and AZFL)
2015: 78.0 (reg and post season)
If they stretch him out, he'll have to be on a 100-120 inning limit, or thereabouts. I'd ignore the critics and just put him in AAA next season. They burned an option on him already, but regardless, let him work on his secondary stuff while being in an environment where his innings/pitch count can be controlled. Expecting John Gibbons to show any type of restraint with a young pitcher would be like putting a loaded gun in the hands of a monkey and expecting no one to get shot. Keep Osuna away from Gibby in 2016 if you plan to stretch him out. Stretching him out in the Majors when he spent an entire year throwing fastballs in the pen would be foolish.
I'm not sure about Hendriks in the rotation. He's a great reliever regardless of how the manager uses him. Just give him more of a role in the pen next season and see what happens. But if they want to see if his velocity spike can be somewhat sustained in longer outings, then give it a shot.