Hardware wise all I've noticed is that the AP51 has a stronger processor and slightly more memory.
I personally added the Mk 3 features to the fon v1 way back as a POC, the only problem that arose was when network sniffing/MiTM, if the processor hit 100% or the memory hit 100% the fon would reboot, and lose the data.
I believe the Mark 3, interface and scripts should work on the fon.
The problem is that its complex and a lot of work, if you dont know what your doing its difficult to debug and fix.
Darren and the Team are doing an amazing job, at simplifying this processes so that eventually it will be available to everyone.
Its understandable that we all have other projects/jobs that take up most of our time.
We all just have to be patient.