I have made that same trip, Virginia to Beira several times in a Seneca I and more often in a Cheiftan. I always stopped for fuel at Maputo (LM) or Inhambane. I can't remember exactly the fuel burn at altitude but full to tanks dry in a PA34-200 of 5 hours does seem to ring a bell.
I worked in aviation for 4 years in Moz and am still sceptical when reading IACM reports - with the best will in the world they do tend to be fixated on admin issues rather than engineering/operational matters.
It is not impossible that neither Maputo nor Inhambane had Avgas, despite notification to the contrary. Occasionally there would be drum stock at Vilanculos but this tended to be pre-positioned by local operators. I can certainly imagine the scenario of an intended landing at Maputo to be told on initial contact that the airport was closed or that there was no Avgas available - press on to Inhambane and be told the same. You are then snookered!
The local governor could close the airport on a whim, he could also direct that no more Avgas be sold. This info would not be NOTAM'd therefore not available in Durban (it is unlikely that other airfields in Moz, would know this either!) It is certainly info that could be overlooked by IACM.
The country suffered from thirty (30) years of civil war that effectively destroyed all of the infrastructure.
I have met Mr Howell, here in Norfolk a couple of times - a very interesting and entertaining chap. Whatever the outcome it is another tragedy in Mozambique, a country that has seen more thanits fair share.