Have been racking my brain to get this working. Any help and tips would be appreciated.
Can I suggest that the AE Gen6 might actually have problems.
And especially if the computers you are connecting to are located on the wireless extend units.
How I would work from here is to save your present settings.. on all the units.. (export configuration).
Then go back to a single AE as router.. reset to factory and start over with just one computer plugged in by ethernet. And try your various forwardings on it.
If any of them fail.. take the "Airport Extreme Extended" which you have in the network as wireless extender and put it into the place as main router.
Let me suggest further you take its firmware back to 7.6.1 which seems to handle port mapping better.
This is easy to do.. hold down the option key and click on the version in the summary page.
Factory reset it and do a clean setup with simple short names.
Now see how your port forwarding goes..
Make the network as simple as possible.
Sorry it is a load of work.. you can easily recover to where you were.. by using the import configuration file.. so any messing around you did is fixed..
But my experience with network issues.. suggest start simple.. if it works then move ahead with one extend unit and put a computer on there and test again..
If it fails, then no more complicated setup will work until you figure out the issues.. and I do encourage you to try something other than a Gen6 AE.. (or Gen5 TC which is the same).. even grab another brand router so you have a comparison.
Remember too as scott has suggested any port already allocated cannot be used.. that also means NAT-PMP ports allocated by the router or used internally cannot be used.. so leave off BTMM in the testing phase.. simple as possible.. then you have a real chance to see at what point the problems come in.