NIL deals have no stipulation to be annual. Take Paige's deal with Gatorade, it is multi year. The collective is essentially the same as Gatorade, so they have the opportunity to make a deal as long as they please, but one would have to imagine that it has stipulations for playing on that particular team if it is a team's collective.
On your side note, that is definitely going to be a monster problem. Since the players are not getting w2s, and a ton of the smaller to medium deals probably do not involve some agent, then there is probably a huge hole in the kid's circle in that area of advice.