"If it is the 'greatest desire' that determines the will, then how could anything done said to be 'forced'?"

You just answered your own question. Man's choices are determined by his desires.

That's how we describe the animals, only instead of desires, we call them "instincts."

Man is not determined by desire. Man is influenced by desire. Man is also influenced by reason. Man has a brain, and that brain is supposed to control his desires. I think a complete picture of man includes his desires, his will, and something else - call it his heart. It's the place for stable sentitments, and a liaison between logic and desire (brain, and stomach if you like). Patriotism, for example is often both illogical and inconvenient. But it keeps the soldier in the battle when he wants to run away.

CS Lewis give a great example of what I'm trying to say. Say you're walking near a lake, and hear some splashing and a cry for help. Two desires should immediately well up: desire to help the person, and desire to stay safe on dry land. There may be more, but the point is that the greatest desire is probably to stay safe. Yet the right thing to do would be to overcome that desire.

The thing that stands above these desires cannot be a desire itself.