Bryan,
Great write-up!!
Your post sent me down a rabbit hole and I ended up reading this lengthy but very enjoyable SI article on Grange from 1985:
https://www.si.com/vault/1985/09/04/...st-of-all-time
With respect to the signature, I agree that the G looks like that from Grange... But the "e" in Joe looks different. It's tough because most of the handwriting examples I could find online are from his later years, and obviously people change their styles over many years. The e in Joe is upright whereas the other e examples I can find are all heavily slanted. I also looked for examples of how tight Grange wrote his letter m, but again, I didn't find much of anything from the 1920's.
You do make a compelling case though...I hope someone with more handwriting expertise will chime in!
Steve