If you have re-read the posts you will already have seen that the P645N doesn't have mirror lock-up but someone who has experience of both the 645N and 645 Nii says that mirror lock-up is not necessary.
I too have a P645N and haven't yet found that mirror lock-up to be necessary. The 645Nii is more expensive and other than mirror lock-up has nothing extra/better that I can see
Have a look at Ken Rockwell's site. He has a review of the P645N which I think is accurate.