Yes, release everything, because I believe that having an economy around the FP2 can in turn boost the sales of the phone itself. See e.g. Raspberry Pi, Arduino, or even Apple’s “Made for iPhone”.
If I think about it, Fairphone should test the third-party-hardware and give them a "Made for FP2"label.
E.g. this physical keyboard for the Jolla phone. They have something similar to FP2 (back cover with interface, called “the other half”) and provide a developer kit for it.
this kind of freedom allow for example to develop a third part better or different main or front camera, another kind of usb port, a solar powered back cover, etc.
Actually I already saw a FP-compatible(?) cover on a (likely not very fair) chinese website. I think allowing the development of 3rd party hardware would help goodwilling people make cool stuff (others won’t care).