I'll just give you my thoughts. Any other ideas are of course welcome and please tell me if I got something wrong or if you think otherwise.
2) I think the latter will be more accurate because if you were to calculate the moles (assuming that you use the ideal gas equation), you need the pressure, temperature, and the volume. Therefore, there is a big chance of getting erroneous values as you measure all the three.
3) PV = nRT
V/n= RT/P
If you fail to subtract the water vapor pressure, the pressure of oxygen will be greater than its real value. Therefore, the molar volume will be lower than it should be.