Pedant maths:

“The reason the towline post on canal boats is near midships is that the further forward the post is, the greater the force drawing the bow toward the bank of the canal and the greater the effort the boatman has to exert to counteract this force.”

It's two systems. Do the math for tow angles of 0 & 90°, any loading. Moving the post (load) fore and aft doesn't change the basic geometry of the force vector triangle.

The vessel balances (pivots) on its center of lateral resistance. Like a see-saw, moving the load doesn't change the load. It takes less rudder effort because it's the same load on a shorter lever.

Midships posts produce much the same advantages in turns as nearside bitting in straight runs. Typical of boats, there are advantages and trade-offs to all the options.