In yor opinion, how does QBANK question performance serve as a proxy for test preparedness?
If you are doing all difficulty levels…not good.
What difficulty level would be best?
I think you know the answer to that… 
Seriously though, the easy and intermediate questions can help you with the concepts, which is important…but I wouldn’t count on the actual exam being at that level.
Don’t let Q-bank give you a dangerous false sense of security. I doubt that 70% on Q-bank will translate into 70% on the actual exam. With all of the laughably easy questions on Q-Bank, I would expect 6/7 to be MUCH harder.
I get what you’re saying. 
Yes, but on the same token the length of some of those harder questions does not represent what you see in the real test - so if you mess those up don’t stress. The real exam is much more straight forward in layout - you are not necessarily bombared with all that data as you are on the Q bank.