I cannot find any tech info on this unit so the following are just general comments.

[COLOR="Blue"]
1) measured resistance on the pins on compressor C-R 14.1; C-S 14.7; RS 29 ohms[/COLOR]
This looks OK as far as the two windings adding up when measured in series.
But I do not kn ow what the actual winding resistances should be.
Did you also check that the windings had infinite resistance to the compressor housing?

[COLOR="Blue"]2) Shaking it made only the slightest sound, didn't sound like I've heard on youtube vids... seems to me only the sound of the contacts moving slightly in the case (i.e. what connects to pins)[/COLOR]
A PTC is just a temperature dependant resistor so there should not be any rattle. The rattle is for a start relay.

[COLOR="Blue"]4) Measured 122v on EACH side of the PTC relay connectors C-R and C-S; from what I'd gleaned online was expecting only to have 120v on the C-S side?[/COLOR]
This looks normal to me.
As the PTC resistor heats up it will reduce the current through it to the start winding. But you will still see the voltage there

[COLOR="Blue"]5) PTC makes a click sound at some point, presumably on heat up and cool down. The unit that the C runs through gets very hot... I presume this is the temperature relay component?[/COLOR]
I am not sure what this is.
It could be the thermal protect.

[COLOR="Blue"] There is a conductive disk that is suspended between the contacts for the R and S sides... not sure what this does would like to know. Also, I am not sure I replaced it in same orientation if it is directional.[/COLOR]
The disk is probably the temperature overload device (thermal protect).
They do not depend on orientation.

[COLOR="Blue"]6) Compressor will run, but maybe not always.[/COLOR]
I am not sure what "maybe not always" means.
Since I do not have any tech data on this unit I cannot say if it has auto defrost. If it does then the compressor and ans will not run during defrost which usually lasts for about 30 minutes.

[COLOR="Blue"]Left it for a couple of hours running and it did not cool at all. [/COLOR]
This is the important bit.
If the compressor runs and the unit does not cool down then it is a sealed system problem.
Check the evaporator coils in the unit.
They should get cold and develop a fairly even coating of frost.
If there is just frost in one area (usually where the freon enters them) then it is a sealed system problem.
If the unit has a condenser/compressor fan it should be on whenever the compressor is on and vice versa.
If the unit has an evaporator fan it should be on when the unit is trying to cool down.

[COLOR="Blue"]Compressor on wine cellar was quite hot. When I plugged it back in after stopping it compressor did not come on right away, not sure if that is normal or not.[/COLOR]
It is not normal but perhaps the thermal protect had popped and had to cool down and reset.


[COLOR="Blue"]So... does this sound like a Freon charge issue?[/COLOR]
Yes it sounds like a freon or at least a sealed system problem as long as you are sure that the compressor was running and you got none or minimal cooling.
You can usually hear a compressor but also should be able to feel it vibrating when it is on.
If it is a sealed system problem then it will be an expensive repair as you need a pro. In most cases it makes more sense to put the dollars towards a new unity.

[COLOR="Blue"]Or could it be related to the PTC relay in some way?[/COLOR]
If the compressor runs then it is not a PTC problem as far as the unit cooling.
It may be the cause of the compressor overheating.

[COLOR="Blue"]What other tests can I perform?[/COLOR]
See above