Is the soul, in effect, commanded by karma to participate in future life extensions, or is this always a free will choice?

It is always a free will choice, so it is done willingly by the soul each and every time a person engages in a future life extension. It, again, has the same potential of making things better and adding to learning and growth in the bargain. So, in effect, it is not seen as a penalty at all but rather another go-round, another set of experiences and, in fact, another lifetime. This is always a blessing because it contains inherently many, many possibilities and much more benefit than loss. This you will see eventually when you emerge from the difficulties of your existence and its history, and see all of the wisdom you have gained from taking on the complexity, and many challenges it has brought to you. That is the prize—it is having won the fight. Your trophy is a testament not simply to the fact you are a winner, but it is the realization you have achieved the most difficult of goals despite the many blows, the bruisings, the pain, the fatigue, and the suffering all along the way with the training and preparation, let alone the actual boxing match itself, so to speak, so having the opportunity to engage in life as many ways as one chooses is a blessing and a privilege.