The downside is that when you arrive at the Moon, you need the Moon to be close to a node with the final orbit.
Exactly. This means it is not really applicable for large inclination changes to low Earh orbits as you have to spend lots of energy for the plane change to arrive at the Moon at the right time. This energy could be used for methods 1a or 1b instead and still be more efficient I think.
Thanks for posting the formulas for the calculations, I had found them on Wikipedia before but did not get to grips with them yet.