Has to be done with the body off for them to be able to fit the test rig in. If its a kit where the body cant be removed then it cant be tested so i dont know what happens there.
It gets done with the seats in (they are destroyed), im not sure about the belts but i think they need to be installed.
They said it may or may not deform the chassis depending on how mounts are designed. If you test the actual chassis it could be weakend and not work when needed in real life.
The RTA say they dont want you to have to destroy the chassis they will accept testing on a prototype. That doesnt help though cause the test is super expensive and you need to virtually build a whole new chassis so it can be held in place for the test.
So $11.5k for testing, $500 for some seats to break, $200 for seat belts, $1k to build a prototype. So $13.5k
then add brake test, noise test and engineering inspections and you are getting near $20k for engineering one car plus you dont know if another test will be required when you have the inspection. eg weld testing, additional tests in ADRs etc.