Dunbar and Price are around the same height, have similarly-colored hair, and wear the same kind of jacket with the fur-lined collar. So when the others throw Price out of the barracks after finding out he's The Mole, and the Germans shoot him down, it's understandable they'd think they had finally gotten Dunbar, instead of Price.
When Sefton reveals Price as the spy, the others are pretty quick to believe him. Most likely, they were already starting to doubt Price's allegiance, since, as Price himself points out, things have started to fall apart ever since he was elected barracks security.
Price being so suspicious of Sefton isn't just concern for Sefton being the mole: with all of the privileges that Sefton gets, Price is likely worried that Sefton is being a better mole than he is and Price could be in danger of being replaced. That's why he questions him so much in the wake of Manfriedi and Johnson's deaths: to see if Sefton is a well rewarded informant who's not very subtle at the job or just a cynical jerkass.
What would be done to the POWs in a prison camp where the Geneva convention is only paid lip service, when the commandant finds out that they had the mole killed and helped two prisoners escape?
It's implied that the commandant can't punish the POWs without revealing who the mole was (the Swiss representative of the Red Cross just reminded him earlier in the film against harsh tactics). He's stuck with the story that Price was an escaping POW.
In a deleted/alternate ending, all of the POWs later escaped through the hole Sefton and Dunbar cut in the gate, and later marched through the forest, all while whistling, "When Johnny Comes Marching Home Again".
