A model number may help others help you

Here are your parts
Replacement parts for WHIRLPOOL KEBS107DAL4 | AppliancePartsPros.com

Here is the tech sheet
http://www.servicematters.com/docs/wiring/Tech%20Sheet%20-%204454769.pdf

Here is a manual, although it is for newer units it may come in handy
https://secured.whirlpool.com/Service/SrvTechAdm.nsf/2cd44500d572193285256a45004fd9d6/6342e2edf7772b6e852573440054cfc6/$FILE/4317418%20Single%20Wall%20Oven.pdf

Try removing power from the unit for 10 minutes or so to see if the control board (EOC) will reset.

The error you see is related to cavity size but this does not make sense as it is jumper settings and they are not going to change. This points to a bad control board.

Does sound like you have two different problems. One the error code and the second the buttons not working.

Unfortunately there is no info available for the keypad which would allow you to check it with a meter. Buttons not working can be either the keypad itself or the control board.
You may want to try the following. On most keyboards a number of keys will use a common circuit (track) and that may be why more than one key is not working.
Connector Cleaning / Checking
Unplug the unit
Open it up
Remove the ribbon cable from the keypad to the control board at the control board.
Clean the edge connector.
If it is the type where there is not a connector just the insulation is removed, it can be cleaned with a pencil eraser. Do not use an ink eraser as it is too abrasive.
Check the tracks in the ribbon cable for hairline cracks. A magnifying glass helps here.
Would not hurt to clean the connector on the board. Use electronics contact cleaner or isopropyl alcohol and a tooth brush. Be gentle and do not reconnect till all the solvent evaporates.
Reconnect everything and give it a try.