Thinking about leaving industry job and taking up a postdoc
I am in a different field (maths), but I know the stories of several people who went to work in unrelated industry jobs after their PhD, only to decide after a few months to return to academia. It never seemed to have hurt them, but there is of course a danger of survivorship bias, as I never got to know those who failed.

Since it does not seem like you actively burned bridges but only declined offers, I would simply approach the professor with a bit of humility. You made a mistake, but you are willing to correct it and six months is a short time in academia anyway. To add to that there is less danger of losing you to industry now than there is for a post-doc who hasn't tried it yet.

Concerning the "dark side" jokes, with most people they are merely that, namely jokes, so I wouldn't worry that much. They didn't really move away from you, you did move away from them. After all, with you in industry, there were simply no more research collaborations to be had and less of common interests.