Uhmmm...50mm, that's quite a bit to bend it back to. I know that you said that the bend is not a sharp kink, but is it concentrated at short legnths of the stays? It will make it more questionable if you should bend it back if it is compared to a bend that was more gradua,l along longer legnths of the stays. I think that would be the deciding factor. if an attempt should even be tried.
Like you said though, the general rule out there is Al frames should not be cold set, so I suspect that you are on your own caus I think that all responsible bike shops and frame builders will not even try to fix it too.
Chombi