Hi Marie,
1) don't take kris words in a bad way, there are some situations when you really need to have processes in the same session in order to handle interprocesses communications. It doesn't means that there are buggy behaviors when you run two or more processes in the same ksession.
2 and 3) Could be expensive if you think about persisting multiple session and based on the process you need to know which one load again. You will need to implement a mechanism to keep track which session -> which process
I personally think that you will start to understand more the posibilities that you have when you start looking at the persistence configurations and how they work.
Greetings.