Finding a suitable nonce is a lot like reverse phonebook search, and Grover's algorithm operates in order sqrt(N) guesses, instead of the classical N guesses. Assuming this is in fact the best way to use a quantum computer for mining, this has a curious effect. It means that if the difficulty quadrupled, it would take only twice as long to find a suitable nonce
Doubt it.
How I understand QC its just a matter of what kind of tradeoff you have to take to solve a problem with a computer of a particular size. And if you can even make such a trade-off (currently we can not)
As quantum computers get larger the trade-off can be reduced to the point where it is none existent.