The overall shape of the shell looks pretty good and curling of the rear visor, or neck guard on an officer's helmet isn't unusual (thin leather). PERSONALLY, I wouldn't worry at all about the neck guard, because bending it back outward might do some damage that you don't want. To me, it looks as if you just set it on a form (other than a styrofoam wig head) and allow it to 'relax' back into shape over time (years, even a decade isn't too long), you'll be good.
Collectors understand that the neck guards on these will curl and it's not as much of a detriment to the helmet as one forced back into place and cracked or otherwise damaged. Again, if you want to straighten it, it will have to be gently pushed back into shape over years.
Of course, these are just MY opinions, I'm more of a 'hands off' type of person when it comes to minor restorations, others may disagree or offer suggestions that work for them. I only restore when my efforts have very little chance of damaging an already messed up helmet and it's been many, many, many years since I did that.
Your shell looks to be pretty good and with a little cleaning should be good as is. I'd like to see it with the furniture, or hardware on it.
Ron