Hypothetically …
Let’s assume all of the key problems you mentioned weren’t there in Android 6 because of Qualcomm’s work in their blobs. We at least know that there’s nothing coming from Qualcomm anymore and this is the cause for at least some of the issues, even if Fairphone still wouldn’t say yet which issues exactly.
Let’s assume even if Qualcomm are not supporting Android 7 on their Snapdragon 801, they are not giving the sources of their Android 6 blobs - still important intellectual property as they might see it - to anybody … at least not for a reasonable price … and not to Fairphone either.
Let’s assume Fairphone developers right now are doing their best to somehow find ways to fix the Android 7 problems while still having to work with the “black box” Android 6 blobs, and without losing the important Google certification.
(By the way, while the LineageOS community can approach things differently, LineageOS on the Fairphone 2 also suffers from at least the new camera bugs and the lack of Dual-SIM control).
Let’s assume that much … but you could still not say in detail from the outside where efforts are perhaps stuck and you could not know whether an additional developer would actually help.
I would be in favour of instead funding daily progress reports about what’s being done currently by Fairphone developers … if such “surveillance” wouldn’t almost certainly poison the working environment and have adverse effects.
(Who of us gets stuff done better/ faster/ at all when somebody constantly is looking over your shoulder, constantly demanding to know how much progress you make 
?)
So, bottom line … Fairphone would have to come out and say whether they need help, and what exactly they need help with.
And then somebody could step forward and say “Hey, I could help with that, because I have already done [insert helpful expertise here], or because I have some ideas how to tackle this or that which you perhaps didn’t try yet. Could somebody fund me for the time I’m needed with this?”
And then there would be sense in a funding effort … if Fairphone aren’t able or willing to fund it themselves.