He demonstrates a certain amount of free will in terms of what he says and his memories, so the control over him is not complete. They could have just taken over to the point of blocking memories of the past and substituting their own constructed past so he'd remember growing up on their planet. They didn't, so they did allow him to keep an awareness of his life and even to talk about it with the constructed characters in the simulation.
With that awareness, especially in a person with a will as strong as Jean-Luc Picards, freedom of movement and thought would be extremely important. There'd have to be freedom of two different types. The first would be freedom of movement. He'd have to be able to explore and not feel trapped. He'd also have to be able to accept or decline invitations or actions. For instance, if they invited him to their planet's equivalent of WWF, it's like Jean-Luc Picard would have insisted on declining so he could read a book on poetry.
It would be like our Interstates. You have a general route like, say, I-95 going from Maine to Florida, but you might get off on the side roads, or stop at rest stops, or take extra time to stop in Washington, D.C. on the way down to Florida so you can see the museums, but, in the long run, the road and the side roads all lead to the same place (in this case, the launching of the probe).
It's likely they had optional routines in the program to set the probe launch event at different points. If, after five years, he was getting too restless, they could have led up to the probe launch at that time, but the longer the time in the scenario, the better he'd remember the people and the culture.
Notice that he arrived in the scenario after he would have had children and all the characters were in place already (other than his grandchild, which was the result of other characters' behavior and not his). So the scenario didn't depend too heavily on him marrying or having children or picking a job or career.
As long as he stayed in the community, there was still ample "play" in the scenario for him to have the experiences they wanted him to have.
The part I never figured out is how wide a geographical area they let him explore. I would think they'd have wanted to have as much of the planet mapped out for a visitor like him as they could.
They also seemed to have other characters ready to work with him and to calm him down if he were to get unruly. Notice the others didn't overreact to him when he talked about life on the Enterprise but seemed to know more about guiding him than pushing him around. I'm sure that's a pattern that was ready to come into play anywhere in the simulation when needed. (And if they had to take him to the hospital and give him a psychotropic drug, I'm sure that would work much better than restricting his choice and not letting him go places or forcing him to play along - especially with any strong willed person.)