How to gauge resistance on parallel circuit with step down buck regulator
This is not a complete answer but meant to clear up a couple of misconceptions that appear in your question.


If the load on your 5 V converter is 70 mA, the converter will only produce 70 mA. The 300 mA is a maximum rating that doesn't affect how much current is drawn from the 12 V supply.
If you reduce the input voltage to the buck converter, it will actually increase the current it draws. This is because in order to produce Vout from Vin with conservation of energy you have


$$I_{out} \cdot V_{out} = E \cdot I_{in} \cdot V_{in} $$

(\$E\$ is an efficiency factor)

Since the regulator is trying to keep Vout the same, and the load will require the same current as long as Vout is the same, Iin will have to go up when Vin goes down.