Thermochemistry questions regarding enthalpy
You should not separate the species on the left and assign enthalpy variations to each of them. On Scenario 1, you say that 1 mol of methane reacts with 2 mols of oxygen, and the variation of enthalpy for this is −889 kJ (since it is 1 mol of methane). If you had 2 mols of methane, and 4 mols of oxygen, $\Delta$H would be 2*(-889) kJ, and so on. As you mentioned, if only 1 mol of oxygen were available, only 0.5 mol of methane would react and $\Delta$H would be 0.5*(-889) kJ
So, for Scenario 2, you said it was given that

the oxygen produces 400kJ of energy

This probably means that, for each mol of oxygen that reacts with ethanol, 400 kJ of energy is released. So yes, if you wanted the $\Delta$H for the reaction, you should multiply by 3 and get that $\Delta$H = -1200 kJ mol-1.
This is in agreement with the measured molar enthalpy of combustion of ethanol (-1211 kJ mol-1)