What would the interlock do, and/or connect to, to do somethig that the software wouldn't do already as part of controlling the relay, and turning off the relay if the temperature got too hot?
I would assume the interlock would be to determine if oven door is open/closed so you don't turn on the heater if door is open.
You could reclaim 4 pins back (if needed) by using 4 bit data bus to LCD instead of 8 bit.