Were do you get your info from? According to Arne Sagen the arrangement for pumping out the hold/ctr.tanks was via a tunnel along the centreline in the double bottom:
Thus the double bottom was NOT used to carry oil. The only part of the double bottom which could get oil/gas filled would have been this tunnel.
The problem that was highlighted by Capt.Johnny Eilers was that the inertgas system that was intended to keep the wing tanks, which was used to carry oil but not always cleaned during voyages carrying iron ore, inerted:
These combination vessels were phased out after the three accident;, Berge Istra/Vanga and Derbyshire.They became used as pure Ore Carriers, or Tankers.
You are right, usually only every second hold are used when loading heavy ore cargo, which was probably the case on the Stellar Daisy as well.
According to a report the Stellar Daisy was carrying "fine iron ore", which is much more exposed to liquefaction than the normal "course" iron ore. Several accidents caused by liquefaction of other ore cargos has occurred lately due to too high humidity content in the cargo when loaded.