is the rear hanger / dropout part of the frame? in case of braking it can it be repaired?
There is no reason to think a "short" chainstay would cause that, the differences in lengths of CS among bikes is fairly minimal, even between wheelsizes. The issue of the chain skipping down the cog is fairly common, and has a lot more to do with chainline. Bring your cranks inboard if possible. Otherwise, just don't backpedal while in the lowest gear...Anybody running 1x11 have issues backpedaling in the largest cog? I want to know if the chainstays are too short.
A short chainstay length will make the angles slightly more significant. But unless there's a several inch difference in chainstay length, it won't be enough of a difference to matter between any 2 bikes.There is no reason to think a "short" chainstay would cause that