Assuming the force is placed at a distance L from the support, then the moment of the Force P with respect to the foundation will be $ M = P\cdot L$.
If the horizontal square tubing tilts by an angle $
theta$ then the will be equal to $M = O \cdot L\cdot \cos\theta$
For angle up to 10 degrees, the $\cos \theta$ has a minimal change ($\cos 10^o =0.984$), so ...
Q: Can I assume the bending moment is simply the load times the distance from the load to the center of the column?
A: Yes. You have to make the assumption that the cantilever and the short tubing are fitted tightly, so the resultant forces remain in the same locations throughout the process. That is, there are no excessive geometry changes at the beam-...
You should remove the rocks by explosion or rolling them away from the foundation.
A site with large boulders is usually a young geology. deferential settlement and high underground water table are likely.
You should get a geologist check it.