You have assigned the question to yourself, no one else can answer it now we can only comment on it.

The probability of failure for the mbed is going to be farily bad, you have two CPUs, a voltage regulator and some didoes, any of them could fail and take the system down. Not to mention a number of physical connections. Plus if this is something you have handy what's it's history? Can you be sure that every single time it was taken out of it's anti-static bag that anyone who touched it took all of the anti-static precautions?

In other words for something safety critical using overly complex modules and parts you have sitting around is not generally a good plan.

For a probability of failure number you should be able to get one for the base CPU from NXP. You then need to add the second controller and the power supply components, use the schematic or BoM for the board for details of those parts. Once you work it all out it's going to be a lot worse than for a CPU on it's own because there is a lot more going on.

Hello Andy! I actually have a new mbed. Got it quite a while ago and only used it once. I agree with your comments! It is not ideal for the safety critical context. It is only one small part of my project (quite a bit of redundancy also). Thank you for the guidance for finding the total number. I will look into it.

Cheers