EDIT: This post previously had 2 questions but @DavidSupportsMonica gave an acceptable answer in the comments so I deleted the now solved question.
I wanted to know whether:
Suppose that someone's priority date is current (read, their chance have finally come in the queue), are they legally required to take that green card and start residing in the US straightaway? What if they actually arrive in the US after 1-2 years or say a much bigger number, 5-7 years.
Asking because my circumstances are a bit strange.