How I understood the AMA and some posts from an unhappy FP1 user here, they use an external repair center. So most phones will have to be send to them and they decide on how to proceed. Also they have to ensure that is was not the users fault, I assume. (@anon48893843 called it “been swimming in river” 
)
But @anon48893843 also told me “… Since having a repair center in the Netherlands is expensive (Dutch labour costs…) you want to make to make sure that when a phone is sent to our repair center, it really needs to go there.”
So it’s all a bit confusing, so I guess they don’t have a good workflow establish for all this already. @anon48893843 called it “Not because FP2 is a bad product, but the modularity, the reverse logistics coming with the modularity and the complexity of our systems and processes overall made it difficult to keep up.”
The did not told any details so it’s difficult to speculate about this or … and that’s even worse … help them. Using Zendesk and having an external repair center sounds pretty good to me. The only open question I still have: What kind of problems do all the phones they get back mostly have? Is it just the “normal” module/phone error rate they couldn’t handle? I personally don’t think so … it looks like they have to handle a big mix of software/firmware/module related issues.
An answer to this could also help answering your questions above. If the camera module is known to be broken in a certain unrepairable way, the phone does not really need to be sent in, all you need is a new module.