Hi Carrier:
I can only offer you my personal opinion, which is that because the Twin Otter is a fixed gear aircraft - and one with very robust fixed gear at that - I think the risks arising from carrying fuel in the belly of the aircraft are pretty minimal.
If a single main gear leg collapses, the aircraft will pivot about the wingtip, the belly does not hit the ground if the wing is not deformed. If the wing does deform, well, any fuel that would have been carried in the wing would spill, thus negating the benefit of carrying fuel out there.
I appreciate your point that fuel carried in the belly could be a hazard in the event of a gear collapse or gear-up landing of a retractable gear aircraft, but I don't think much risk of that exists with the Twin Otter design.
I think the reason that most aircraft (that have wing fuel storage) put the fuel way out at the end of the wing has to do with reducing wing bending moment, not moving the fuel away from the passengers. If you purchase a Twin Otter with optional extended range wing fuel tanks, they are way out at the very end of the wing - in order to reduce the wing bending moment.
Michael