Yes, it's possible. But those doing the nibbing could just as easily nib the nominations too.

Yes but nibbers are anonymous whereas anyone can nominate a piece. What happens if there is a discrepancy of opinion?

Remember what it says in the bible: blessed are the nibbers and nommers for they shall see God. 

Only God knows who the nibbers are.

Yes. They seem to come and go like thieves in the night.

Also, it depends when you post. I think we should understand just one person's opinion. I have had some that have been read so many times more than others and never nibbed or nominated. Some on reposting almost ignored when nomed or nibbed before.

I don't repost for any other reason than to have new audience and fresh input but some I will never change and that's the way of it.

I find on re-reading older poems, glaring flaws stand out for me, so gives me a chance to see if I can improve it but in all poetry, it's a subjective thing.