A full model number would help.
Find your model number - AppliancePartsPros.com

See the attachment for the tech sheet.

I am assuming that the unit stays in preheat and never comes ready.
I am not sure how/where you tested the upper oven relay.
Also by upper oven relay I am assuming you mean the Double line break relay.

Unfortunately the wiring diagram does not give pin out numbers for this, just shows two Red wires at the control board.
If the relay is actually closing then you should see 0 volts across it when in bake or broil.
If it does not close you will see 240 volts.

It can get a bit confusing if referencing to ground.
The common (lower R line in the wiring diagram) goes directly to L2 so should always have 120 volts on it.
The NO (normally open) contact (upper R line in the wiring diagram ) goes to the elements.
With the oven off this line should be 0 ohms.
With the oven on it should have 120 volts on it.
Now this is where it gets complicated.
But you do not know if the 120 volts is the L2 or the L1 voltage.
If the relay closes then you see the L2 voltage but if it does not close you also will see 120 volts but this is the L1 voltage because either the bake relay or the broil relay is closed. The L1 voltage then feeds through that relay, the oven element and the thermal fuse and you see it at R of the line break relay.

I hope I explained the above OK

[COLOR="Blue"]Oven shows preheat timer countdown, and will count down, then show its ready. It THINKS its heating up, but neither bake nor broil elements will get hot.[/COLOR]
This points towards either an oven sensor or a control board problem.
Below is info on how to check the oven sensor, take a look at 7, 8, 20 and 21.
http://www.applianceaid.com/ele...-range-repair-topics.php

If it was just a relay not closing then the unit would never get any heat and it should stay in pre-heat.
So if the sensor is OK then I would replace the control board.

Note that if you get the control board from AppliancePartsPros and it does not fix the problem then you could send it back. See their return policy.

[COLOR="Blue"]
So, what test should I run next?[/COLOR]
I would check the oven sensor.

[COLOR="Blue"]Could you translate your explanation of L1 and L2 voltage into specific test(s) I could do?[/COLOR]
With the oven showing it is ready I do not believe you have to follow this diagnostic path.
FYI, just i just in case you do not know how the 240 works.
Your house voltage is actually two 120 volt supplies (L1 and L2) that share a common Neutral. They are 180 degrees out of phase so when one is at positive 120 volts, the other is at negative 120 volts. therefore there is 240 volts between them (L1 to L2).

[COLOR="Blue"] Anything else to test or inspect, either other parts of the control board, or other oven components?[/COLOR]
See the start of my post.

[COLOR="Blue"]If not the thermal fuse, temp sensor, or elements, what else is there besides the control board that could cause this issue?[/COLOR]
Just the wiring between the devices is left.

[COLOR="Blue"]I'm stuck for a next step, and at the very least I'd like to confirm the board is bad before replacing it - they're not cheap![/COLOR]
If the sensor and wiring is OK then odds are very high that it is the board.
There is no good way to actually test the board as schematics are not available so it is done by the process of elimination.

You may want to give AppliancePartsPros a call.
If the board does not fix your problem you can return it.
Please see their return policy.

Most places will not let you return electronic parts.