The questions are not usually linked to teach other, just all to the scenario. There were a few questions in quant where you needed to build on the questions, but, thankfully, that was not usually the case. A lot of the item sets would have one or two questions that were not even directly contingent on the scenario, but were theoretical questions based on the topic involved (but not the data).
LII is definitely harder than LI ( I wrote LII this year, so I know whereof I speak), but I agree with jamespucyk's friend that it is largely a factor of more material. It also requires a deeper level of understanding, as the questions tend to be more complex and require a synthesis of different LOS's. The absolute best preparation for starting the LII material is a very good grasp of LI. If you are shaky on the LI material, LII will be a much bigger challenge.
To be honest, the derivatives and quant were not that difficult conceptually. I think that the people who have the most diffculty with derivatives, ironically, seem to be people with a deep background in math. They concentrate too much on the complicated formulas rather than just trying to understand what the product is and how to break it down into it's composite pieces. I truly believe that if you can understand a simple call, put, stock and bond, you can pretty much figure out any derivative.
The toughest parts for me were fixed income and FSA. Fixed income just because of the information overload, and FSA because of all the interrelationships.