Yes! 100%. The longer you wait the harder it will be to stop.
Yes! Absolutely. If the regen doesn't work then it's not safe to drive the truck down the hill. It is equivalent to an ICE truck losing the drive shaft or differential so the engine can't be used for braking.
Yes. I imagine that not riding the brakes down a long hill is already an integral part of training people to drive trucks. I believe there are warnings for car drivers to not ride the brakes on long downhill slopes (like Mount Washington). The key thing is to not ride the brakes which pertains to all vehicles regardless of the method of propulsion.