Here are your parts
Replacement parts for WHIRLPOOL KEHC309JSS4 | AppliancePartsPros.com

Here is the tech sheet
http://www.servicematters.com/d...0Sheet%20-%204453233.pdf

Here is a manual which may be helpful
https://secured.whirlpoo...539256/$FILE/MWCOMBO.PDF

What's the problem?
By the sound of it the service tech is the biggest problem. Any person who replaces parts and then does not check the unit is not much of a technician and it does not sound like a check was done

technician says thermostat ok, so the poor heating (100degrees) low would have to be the control panel.
I am assuming that the oven does preheat and come ready.
Check that all the elements are OK looks like the unit uses Bake, Inner Broil and Outer Broil for preheat but only the Bake and Outer Broil for baking.
Perhaps the Bake or Outer Broil is shot and the unit cannot maintain the temperature. Usually during bake they switch between the elements rather than have them all on at once but I could not confirm this.
I would re-check the thermostat. Here is a good site, check out 7, 8, 20 and 21.
http://www.applianceaid.com/elecrange.html

Replaced control panel and key pad would not work. Says we need a new key pad. Replaced.
I hate to diss a tech without actually being there but this sounds like he/she messed something up when disconnecting the keypad from the control board re: did not unlock it before removal see page 2-3 in the manual or something like this.

This appliance company recommended by Kitchen Aid.
Then get back in touch with Kitchen Aid and raise hell.