No, the MO GOP can’t force him out in any legal way. And due to state “sore loser” laws, the runners up in the primary (Steelman, ect.)cannot run as third-party candidates. So, they would have to find someone else, and they launch the mother of all campaign blitzes in order to stand a chance of pulling of a successful third-party candidate backed by the Republicans and conservatives.
Reminds me of the a$$hole ethically-charged Republican who refused to quit the Colorado Gov. race, forcing Tom Tancredo to run 3rd party, and losing to the open-borders, homo-loving Democrat Hickenlooper.