You see, these are the questions I would like to learn to answer.
My first thought would be that a collar is not an appropriate tool for walking a turtle, as you have found. Something more able to deal with the coming and going of said turtle's head would be better. I would think a tek screw into the shell to mount a hook to, that you could clip your leash to would work.
The head shyness isn't somthing I have any clue about. Again, my first thought would be to attack this problem from the other side, so to speak. To make Tortilla think that whatever it was trying to hide from was behind it, stick your finger up it's bum. That should get it's head to pop out. A few times at doing this would probably teach it that retreat isn't the best defence.
Depending on how Tortilla reacts to your finger should also give you an indication of whether it's a he or she. With fervour and eagerness might suggest a female.
Or gay male.
