Nev: I'm not trying to be a smart$rse here, nor an impromptu crash investigator, but I have had a very decent opportunity to examine a Jabiru that played dead ants..
I would hazard a guess that it hit with some forward velocity, but not a lot, and an attitude of around 200 degrees from normal, possibly a bit more. The separation of the engine from the firewall suggests to me that the brunt of the impact was taken by the engine; knowing the fin structure fairly well, I'll take a bet that it hit with almost no forward motion, as once the fin spar has been fractured, there is very little structure to resist a sliding impact.
I agree with your assessment of the seat-belt restraint capability in the case of a completely vertical impact ( and that's why I intend to put a full six-point harness in my own Jab.) Some forward component would have acted as, essentially, a 'belt-pre-tensioner'. Personally, I believe that a lap-sash arrangement is pretty damn poor restraint; in a forward crash, it twists the spine and neck. My glider flying and car racing all shared four-five-six point harnesses, and though I've never had to utilise their capability, I've seen in my car racing days,some pretty damn huge crashes up close and personal, and the belt arrangement is a huge component of survival.
However: proper belt locating structure is also critical. In the case of the Goulburn Sting crash (an aircraft which in my opinion has the occupant safety of a razor--wire entanglement within a crate of broken glass), the shoulder restraints for the four-point harnesses tore out of their substrate, in a crash that was demonstrably a low-angle contact with the ground. While the disintegration of the airframe around the occupants was horrific to see, the failure of the shoulder-belt restraints just on their own, would quite possibly have resulted in serious/fatal injuries even if the rest of the cabin structure had reasonably withstood the impact.