360 Questions.. pardon my ignorance.
If it should happen that at 5* Idle-timing, the Engine needs a lot of throttle-opening to idle, My guess is that the engine has a mechanical problem, either;
1) a cylinder has gone down, or
2) the cylinder pressure is poor, or
3) the valve-timing is more than just a little off, or
4) the idle AFR is extremely off,
If it should happen at a TRUE 20* Idle-timing, I'd just go straight to cam-timing.
This can be checked close enough without taking the timing cover off, but, one valve cover will need to come off.
The idea is to put the cam on one of either #6 or #1 split overlap and reading the advance off the balancer. It's just that easy.
Split overlap is when, near TDC, both the intake and the exhaust valves are open equally, the one closing while the other is opening. For this test, you don't have to be super accurate, BUT you gotta be sure that the lifters are not bleeding down while yur doing it. And, if your timing chain is really stretched, you gotta do the test with the slack taken out from the "pulling" side.
Ideally, the cam will be in a few degrees advanced.
As to cam-timing
As the cam gets more and more retarded, it takes more and more throttle-opening to idle.
As the cam gets more and more advanced (which does not happen with chain stretch), it takes less and less throttle-opening to idle.
As to ignition timing;
at idle, the engine will always like a bunch of timing, usually into the thirties and more .... at idle.
The more advance it gets, the higher the idle speed will climb.
The more retarded it gets, the lower the idle speed will fall, until it stalls.
>This is why after the Transfer slot to mixture screw adjustment is finalized, Idle rpm is set with Idle-timing. It's the only other option left.
In your case, with the sparkport already being tickled, and the idle-timing pushing 20 degrees, it seems obvious to me, that the engine has a mechanical problem, and the most obvious is cam-timing. But could be other, such as;
1) internal friction, a crank spinning in a very high oil-level, or a dragging convertor, or
2) like very low cylinder pressure, or something I haven't thought of, or
3) like dead cylinders, which could be; bad plugs, bad wires, a bad cap, or even crossed plug wires. Numbers 5 and 7 are occasionally messed up.