WWE is one of the few companies that signs wrestlers to exclusive contracts. Most others, including TNA, allow their wrestlers to perform around the independent scene.
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TNA has changed that policy a few years ago. Now it's almost like WWE policy.
Most of the TNA wrestlers (which means contracted) are
not allowed to appear on PPV, DVD or TV for any other company (which means no ROH, PWG, DGUSA, Japan, etc. for them). BUT, they can wrestle for some other companies that don't put their shows on DVDs, or simply bring them in to wrestle in a pre-show dark matches.
MVP and just a few others though, are allowed to appear on DVD/PPV for other companies, so I guess this is just a bad luck for TNA.
Injuries can happen on any time, fuck it, it's a part of business.
As far PPV main event being in jeopardy, there's always a way around it, hell to make it even 10x better main event. King of the Mountain sounds like a great solution, and can easily be set up at the top of the PPV.