ChristiaanJ,
I do believe you will have to show all the diagrams and math. I note with interest how changing the coordinate system will change the weight of the aircraft. Will the local gravitational acceleration be altered through changing the coordinate system? If not, I regret not having discovered this while I was still doing mass properties!
As for the relevance of VTOL, I trust it you do not contest the fact that lift will be zero when climbing vertically? If we assume you are right and that lift does indeed initially decrease with increasing climb angle, at which climb angle will lift start decreasing with increasing climb angle?
Cheers,
/Fred