Heater control circuit improvement
It appears that the overtemp controller is acting as a supervisor to the main temperature controller. I offer the following comments:


I would be inclined to hard-wire a contact of the over-temperature controller's output, CR28, into the heater circuit so that in the event of a PLC malfunction that the heaters would be switched off.
I would consider adding an NC contact of  CR28 in series with the LT26 reset button to confirm that CR28 has dropped out correctly before reset can be carried out. This is common on e-stop safety circuits and, at least, tests the circuit on power-up.
I would check the overtemp controller's reset to establish whether or not it can be held in reset mode by holding the button in with a wedge, etc. The controller should be using an internal one-shot to prevent this but if it doesn't then you may need to address that risk. Providing the reset one-shot through the PLC might be a solution.
To test this hold the reset in (or jam the button in the depressed state using a toothpick or match), create a fault, clear the fault and see if the overtemp auto-resets or not. (It shouldn't.)
Check that you have open-loop protection in your setup. This means that if the controller doesn't see the expected response in the sensor feedback within a specified time that an alarm is generated and this can be used to cut power via an additional relay. This can protect against situations such as continuously heating while the sensor is hanging out in the air or a short-circuit failure of the heater control relay / SSR.


The need for each additional protection depends on the risk to personnel, property and product which you will have to assess.