Good luck!!!
Some time ago, I tried to get my hunting buddy to allow me to rifle one of more chokes of his .410SXS. That never came to fruition.
Best of luck with your liners, Marrakai - sounds like you've got it figured out fairly well.
The last 6 or so inches of bullet travel behind the muzzles will determine regulation, I think.
Slightly off-centre at the breech for firing pin location will do nothing for regulation, seems to me.