There is no doubt more than one way to chew a Gemara, especially Agaddata, and even more so exotic Agaddata. That it why I didn’t call it proof. I said, “There are certain references that can be taken to be alluding to, or based on, Gilgul.” And sorry, I don’t recall where or who says Gilgul on that Gemara.
The Maharsha you mentioned and pasted (thanks for doing so, btw) does not add or take away from this conversation. He just says why it is a fitting punishment. I am discussing the technical details of the punishment. Will his spine just turn into a snake or is this alluding to a non-physical transformation?