I think you are right with your four questions, although I would rephrase the last one as follows:
How is going to be Fairphone’s approach to open source development?
(yes I am assuming that if we are to be coherent with the statement “If you can’t open it, you don’t own it” FP should have an opens source approach, since FP’s openness has to occur in software as well as in hardware and transparency policies, which you are doing right now quite succesfully)
As I see it, opening FFOS and contributing its code is just one option, but there are more that should be considered, specially if we take into account how difficult (if not impossible) is to update the OS. Please take a look into the poll (and comments) mentioned by @Herve5 to see more options. I also share the oppinion of the 192 people out of 314 voters who are for “Open source / Community developed OS (i.e Firefox OS, Cyanogenmod etc)”. As I explained in this comment , I think that either creating a more community-oriented OS by attract and improving community’s contributions or (better) joining an existing open source project with a consolidated and larger community of developers (like Cyanogen) would allow us to solve the issues regarding updates and maintenance of the OS, as I think the current approach is not sustainable nor scalable at all (we only need to check current figures with updates and time-response we are having within less than a year of development -imagine what will happen when Android 4.2 is really outdated!).