I really do not think that ZH tx is 8 channels as you described. It's also confusing to describe 78P458 as 8 bit but with 10 bit resolution since 8 bit processor can by definition only have 8 bit resolution. I've not look up the specs for the 78P458, but I suspect it is 8 i/o lines with 10 bit A/D and D/A resolution from your description.

The main control chip (The 78P458) has 8 analogue inputs. Four of these go to the gimbals. Four go to the top four dipswitches. So much for using the extra channels.

Again this is just i/o, not channels. The 4 that goes to the switches are just settings that tell the 78P458 how to response/process/what program to run etc, they are not channels. The 4 that goes to the gimbals are A/D converters. The chip reads all 8 i/os and outputs all the info on 1 line only. That is it outputs the data for the 4 channels into 1 line since the info are transmitted in a time based format. This line is also connected to the simulator output. So an easy way to determine how many channels the tx transmittes, hook it up to FMS. My tx that came with walkera is 4 channels even though it looks just like the one for the 22A but has 2 push buttoms instead of toggles. My esky is 5 channels even thought it's advertised as 6. The toggle switchs here do not control additional channels, they just change how the stick responses.

Also the 78 and 36/40 etc tx/rx all work on the same principle. All the info is down converted to 755khz if I recall correctly before it is processed by the rx. Dual conversion would do 2 conversions to IF (intermediate frequency), sinle conversion does it once. But all of them ends up at a final frequency which is in the kilo hertz range.