I understand where you are coming from but the rule was explicitly design to prevent complains that usually go like this:
This robot still sent information to the other one in play whilst being out-of-play!
Oh, and they are also pushing some buttons on that out-of-play robot -- how do we know they are not remotely controlling the other one?
It is therefore quite improbable we would go for just the motor power being turned off as opposed to the whole robot.
With regards to your technical challenges, to be completely honest, I would find a serial cable attached to Arduino still acceptable (even though the other option of attaching Arduino to RPi via GPIO and logic level converter may be a worthy technical challenge in and of it self ).
With regards to slow boot of RPi, I do believe you can get it down to about 20 seconds in total (this is not more than an educated guess based on experiences of others). If you would like to get deeper into this on this forum, please do so by creating a new thread and providing the output of
systemd-analyze critical-chain and
systemd-analyze blame. As RPi is used by many teams, it can be of great value to others as well.
Thanks a ton for keeping the conversation going!