Didn't watch the vid so maybe this is redundant.
You bring up very important points!
Lay under your bus and look up at the "floor slats" you will note that about half of these are attached to the frame with break away clamps. This is a safety feature (until your bus gets too rusty) and something the DOT inspects to insure that the bus can remain in school service.
The purpose of this system is to allow the bus body to slide on the frame during a collision.
So what you don't want to do is attach things like propane lines or seat belts to the bus frame. Attach them to the body and let everything move together.
Re seat rails -not only do these add strength to the body but they are designed to be a secure attachment point, so super handy for tying your furniture and accessories.