Painted yourself into a corner ! You have done everything and you still have a very big problem. Ergo, you HAVEN'T done everything.

First, recognise the fact you haven't done everything.

Second. With that in mind, stop, have a break.
START at the beginning,
When does it go wrong ?
Does it go wrong if the contactor is not powered up ?
Does it go wrong if the loads on the contactor are disconnected ? How are the contacts connected ?
How did you suppress them ?
How did you suppress the contactor coils ?
How did you supress the drive relay ?
What happens if you isolate the contactor from the MCU and activate it ?

.....etc etc.

This is EXACTLY how I was taught to handle faults when I was learning too.

Steve