Honestly, I had passed the last question to my PS consultant. Isn't it controlled by the project setup if project stock is quantity managed? This is what I remember from an exciting business trip to China, where I had to find a solution for a wrong defined project because they wanted to show this inventory in warehouse until it is really consumed.
if you want stock with value when a material is returned from project to neutral warehouse stock, then you must not use UNBW nor NLAG, you have to use a standard material type like ERSA.
The account assignment in the PO decides mainly (interaction with material type definition) whether you create stock or not. E.g. with a UNBW setup you always get stock despite of having an account assignment, that is nature of this material type setting.
For the difference between Q and P see HaTran's answer in Account Assignement P (project) and Q (project make to order)