I disagree with the way that the diagram by OP was originally drawn....
For a start, it would be pretty poor form for an equipment supplier to wire a 3 phase 400V heating circuit in this manner, irregardless of the load or voltage.
What I mean by this, is the fact that under any electrical regulations, this configuration would not fly.
In any country that uses 230/400V, you need to feed your contactor coils with no more than 24VAC and make sure that the wiring through them is "straight through", as in no extra contacts outside of your main contactor.
This is obviously an extra heat circuit, when the weather gets really cold, with it being near 7kW, it seems to me like the contractor low-balled on the heat capacity of the original unit?
