Doesn't make sense that Shears feels forced to go on the mission
The implied threat to turn Shears back over to the American military where he would be prosecuted for impersonating an officer unless he agreed to go on the mission seems pretty hollow. It's a very weak threat when you weigh the prospective outcomes on each side.
Shears had to know that if he went on the mission there was a better than 50 /50 chance he would not come out alive ----on the other hand if he refused to go on the mission and was turned over for prosecution he may not have even been court maritaled and if he was court martialed and convicted (worst case scenario) he might have got anything from a slap on the wrist to a couple of years in the brig. There were a lot of mitigating circumstances in his favor. It would have been in keeping with this character to take a course of action most likely to allow him to come out with a whole skin. He would not have gone on that mission -- he would have said: "I ain't goin --send me back".