[RWebb]

1st - the tough wheel would perhaps toughly slice thru the bulkhead and driver - why would it distribute the load at all?

2nd - thw wheel is not fastened in place - it will just rise up as the crash proceeds -- in fact, if may rise up and be pushed back, striking the driver in the head

this is why engineers do simulations using finite element or other models
it is why they do testing

maybe the spare does help in crash; maybe it hurts; maybe it is neutral for safety

but it will take more than guesses to establish that