Given the impact it's done pretty well, passenger compartment is intact even if the cab and doors are gone. I suspect a 150 would have suffered less damage. Of course with the bodyshell separating it's hard to know if the floor would have been compromised (but it seemed they are designed to sheer to absorb energy)
A Mark 1 would have disintegrated. Just look at the Cowden DEMUS... The Pacer Shell may not be the strongest but it's not as bad as some make out.
Mark 1s were withdrawn for safety reasons.
The main safety concern is using them on the same lines as Virgin Trains, East Coast, East Midlands Trains (London services) and XC services run on. If the heaviest thing that could hit them while in service is a Sprinter then it's much safer.
As has been said before the safest way to use Pacers would be to not use them on services that involve running on a mainline. To allow this to happen Pacers need to be more equally split around Britain. FGW and ATW would need a larger share of them, while Scotrail, EMT, London Midland and maybe even Southern and SWT would need to take on some Pacers.