"yes, the guest would keep the same number if using the same IP address."
That would require a 'cookie' same as signing in, so what's the diff?

"would many people go to these lengths to fool the system?"
My experience online shows that no... and the ones that might, certainly won't do it for very long...

" This might (or might not) deter trolls."
You have to -want- to deter trols in order to actually deter trolls....