This gets to the crux of the problem, he had no idea if the ball was in play and didnt bother checking, he just assumed it wasnt. Despite never checking and making that assumption you have acknowledged he would still take the single had it been offered despite believing the ball was dead. You cant have it both ways. Taking the single despite having no idea if the ball is in play is fine but getting out isnt? sorry it simply doesnt wash with me.
The highlighted part is exactly why the ball is considered in play until both teams have deemed it is not and is exactly why Bairstow should have known better than to leave his crease until the ball was passed from keeper to another player or umpire had called over