Hi Chris, no, I don't think there's anything you've missed. You have interpreted the schematic correctly as far as what pins go to what. The good news is that if PPM is working that tells us at least that the TCB code is functioning and that your receiver is not completely a duff either.
The FS-A8S was my favorite receiver of the FlySky ecosystem, so I know it can work with the TCB. But it could be that they have updated the firmware on the transmitter with extra settings that I don't know about or remember. If (in the transmitter) you set Serial to iBus that should work, but I don't know, maybe you need to re-bind after a change, or maybe for some odd reason it wants the other setting to be PWM. I'm just making random guesses, I really don't remember what the transmitter settings were.
As for the TCB, yes, it starts off looking for sBus, then iBus, then PPM, but if it fails all three then it starts over again and repeats them all in a loop over and over, so if the iBus signal is there, it will pick it up eventually.
I'm willing to bet the answer will end up being something simple.
You could certainly put some diagnostic statements in the TCB code, maybe something that would be even easier and quicker is to download the
IBusBM library. It includes a sample sketch called Ibus_singlemonitor that you can load on to your Mega, connect your receiver as described in the notes at the top of the sketch, and it should spit out the channel values on the Serial Monitor. There is some stuff in there about sensors which you won't need. Anyway at the least it would serve as a sanity check.