[Closed] Shimano RDM8100 to 10 speed compatability?

Has anyone run a 12speed mech on a 10 speed shifter and chain and made it work?

I know the obious answer is, "just change to 12 speed", or "get a 10 speed mech but";

1. I want to future proof incase i do decide at a later date
2. I've just changed my cassett, ring and chain (all 10 speed)

Is the gearing ratio still all in the shifter, or has it changed for the new 12 speed? Also, will a non 12 speed chain fit through the parallelogram?

thanks all..