Most of the time if a player wants out of the NLI, it is granted. Schools don't want kids that don't want to be there, especially if the situation involves a new head coach. They may try and limit who they eventually sign with, but they will usually get a conditional release at worst.
There is really no rule that says a player has to sign an NLI. But signing an NLI protects the players and guarantees the school will have a scholarship for the player. A player could simply enroll in and attend classes in June when the rest of the players report and as long as the school has a scholarship available, no NLI is required. You don't see it ever, but technically, it isn't required.
As far as the early signing period goes, Nebraska is in on several players that are early enrollees, those particular players will not have the luxury of waiting until February since classes start in January. If they wait until after the bowl, which I don't think anyone will do, the early enrollees would have to choose to attend the school anyway, find another school that has an available scholarship and can take them as an EE or wait to sign in February, not enroll early and start up in June with the rest of the class.