I'm a gunshot first camper. Just look at the trajectory of the wound and think about a left handed shooter. Now think about TA being crouched in the shower - and then tell me what you think! With all the manipulation of his body post mortem, his body (and consequently) head wound being washed after being drug back into the shower - and his head being in an upright position - and decomposing for 5 days - I think it's impossible to tell.
I think the gun jammed, which is EXTREMELY PROBABLE (LE here) especially with a small caliber, and possibly old ammunition. I had a .38 revolver that would only fire every other round. Happens more often than you think, depending on a ton of factors!
After the gun jammed, came everything else. No way Jodi would have brought a gun to a knife fight, and used the knife second. She wouldn't have taken that chance.