My understanding is that a given volume of water carries only
so much oxygen and once it has reacted with the inner surfaces of the
heating system then no further corrosion can take place anyway.

That is true for a sealed system as long as there are no leaking joints
which can achieve negative pressure relative to the atmosphere. This
can happen if the system pressure is allowed to fall to much.

For open vented systems, if there is any amount of pump over or suck
down, then oxygen will be rapidly introduced. Even of "perfect", the
header tank is exposed to the air and air will dissolve. Given that
there will be expansion and contraction, there will be introduction of
oxygen.

There should be no reason for vent valves to fail. It would be worth
fitting a different type.

Omitting a good quality inhibitor is inviting an expensive replacement
of radiators later.