1) TWRP replaces the recovery firmware, it doesn't root the phone.
2) Odin is a technician's tool and, like most technician's tools it has no safeguards built into it and relies on the knowledge and the experience of the technician to prevent errors. You can use Odin to do anything to the phone, except damage the actual hardware. But you can totally wipe out all the firmware.
And it's not the way to root a phone.
3) "Return to stock" really doesn't mean anything in the way you're using it, because there's nothing to return to. It's not like turning the page of the book back to return to the words on the last page. You made permanent changes to the phone - you can't "unmake" them. You'll have to find a flashable ROM of the stock firmware and flash it. (I'm afraid that any more fooling around trying to fix things, even if you think you're following the directions carefully, will just result in more damage.)