There are minimum structural strength requirements there, in slightly simplistic terms the occupant and engine must not meet with less than 9g acceleration, whilst smaller point masses such as fire extinguishers need to be retained in at-least 15g.
Is a twin safer in that regard? Probably not to be honest. The engine will stop, or the front of the aeroplane will stop, then the occupants will keep going forward - hopefully being retained if the acceleration is below 9g. To put that in perspective,
But a harness is a good thing, because it does in that initial deceleration generally keep soft pink bodies and sharp and deforming bits of aircraft structure apart from each other (not least the yoke, which probably did much of the damage to this pilot).
What you don't want is anything heavy behind you, so pushers are not necessarily a good idea, nor are fuselage mounted fuel tanks.
Once all the bits have stopped moving, most experience is that there are big enough holes to get out of, and airworthiness standards do require harnesses to be releasable under load.
G