G-CPTN,
The seatbelt is generally like most seatbelts- ie lift the buckle and unclick it goes.
There may be a fixed (to the lap belt part) or removeable shoulder harness which the passengers don't have and they may have struggled with that.
Or they could have for some reason not been able to access the buckle.
But I've never been in that particular aircraft (PJ-SUN) so cannot comment what it had.
There is really no point speculating, we just have to wait for the report.
The Islander and a lot of GA twin pistons, at heavy weights (even below MTOW), will often have trouble maintaining height on one, especially when it is hot outside. Add to that old airframes and engines/props etc and it won't be performing at it's peak.
Ditching the Islander.... not many people would be keen on that. Plenty of other planes out there that would fair a lot better.
"Sittin on the dock of the bay, wastin time.............."