So this is in answer to your question; "On Krishna loka are all of Krishna's pastimes with the living entities controlled by Krishna, or do the living entities have choice in this matter? In other words, is one's rasa governed by strict laws determined by Krishna or is there free choice aside from the free choice of choosing Krishna or maya?"
Your second question, "Why did Arjuna ask Krishna to go to His four armed form rather than His two armed form after Krishna had showed Arjuna His Visvarupa?" Because he understood that Krishna is the original Narayana. After seeing His Visvarupa, Arjuna was accustomed to see Krishna with two hands, but when he saw the Visvarupa, he understood Him the Supreme Personality of Godhead, and as such, he requested Him to revert to the original Form of Narayana. Then, Krishna after showing him the Narayana form, again to revert to His original Krishna form. Krishna explained that Narayana form is not original, but Krishna form in two hands is the original and even the demigods desire to see this form.