Right, you can mix you backstory however you want, even though the c's or a's stick together, these are the only available characters (sadly) but if you have a suggestion I can try to work something out. I would prefer however if you stick with the curernt alternatives. The difficulty affects how harsh the consequnces are and how many hints you get. The higher the difficulty the less help you get and a bad "desicion" will screw you over harder.

Edit: I missread your question slightly. only one option per block yes. so by the end you will have 3 options picked in backstory.