The compressor was replaced on the fridge just about a year ago.
If the compressor was replaced because the unit was running but not cooling (same symptoms). It is very possible that the unit has a slow leak and the compressor did not really need replacing in the first place but they could not find it and concluded that the leak must be in the compressor which is not an unreasonable conclusion.

Is it possible that when the service guy replaced the compressor he didn't seal it properly?
Yes

If so would this be another warranty call even though the fridge is beyond the original 5 year warranty?
I would doubt it, usually warranty is just to get you through the 5 years.
But the compressor may have it's own one year warranty and some places stand behind their work better than others.
I would call whoever did the work and ask, after all you have nothing to loose at this point.
If it is a no go then try Whirlpool.

Here are your parts
Replacement parts for WHIRLPOOL GS6SHAXML00 Refrigerator | AppliancePartsPros.com

Here is a tech sheet
http://www.servicematters.com/d...0Sheet%20-%202255955.pdf

I did notice some frost buildup (~1/16") on the bottom row of the evaporator and also the same amount of buildup on the tube that runs out of the top of the evaporator. The rest of the evaporator is totally frost free. Is this typical buildup
This can be an indicator of a low freon situation if the compressor is running constantly.
Here is a good link with troubleshooting help
http://www.applianceaid.com/refrigerator.html
Check out #20 and then the example pictures at the bottom of the page.
Note the first one does not work.
Your unit should be running without a defrost cycle for at least 2 hours before you check this, you want to give it time to develop some frost.

or is any amount of frost an indicator that the defrost heater is kaput?
No.The evaporator coils must be heavily iced/frosted over to the pint where the evaporator fan cannot pull air through them for it to be a defrost problem.

Also - is the control for the defrost on this fridge electronic?
Yes, it is controlled by the main control board.

Current temps are freezer 33deg / fridge 45deg
If the compressor is running all the time it is trying to lower the temperature but if low on freon it cannot generate enough cooling to do it.

The bad news is that if the unit is low on freon it is an expensive repair job. It must be done by a pro. They must be able to find and repair the leak otherwise it will just leak out again. Also they have to evacuate and recharge the system with freon. Often it make more sense to buy a new unit.

Note that if you are not sure of the units age but feel it is around 5 years check this as the dealed system should have a 5 year warranty.