I take you mean the cooker switch - not sure what that has to do with an RCD.10mm. (It's a 10.5KW Oven)
Yes.
RCD is required. As you can see from the 4th pic, there's a 13A socket attached to the CCU.
The cable you have shown is neither heat resistant nor is it 10mm ² .
The Cooker Connection unit appears to be that single gang space half way down your wall.
Unfortunately the answers or non answers you are giving to the questions raised really do make me think that you do not know what you are doing.Honestly it seems like you're asking questions to be obtuse rather than attempting to help. You can see from the provided pictures and information in the OP the answers to all the questions you asked.
Your perceptions about cable sizes, cable types and RCD's leads me to believe that it would be dangerous to 'encourage' you to do this work yourself.
My advice is to get someone in who knows what they are doing.
BTW not the gas man - no support bracket no isolation switch and most gas engineers I know would not fit flexible gas pipes.
You would use an appropriate multi meterHow do I test the circuit dead?