1. Crocodile with Sirens call has a mandatory action that says it MUST move. But it doesn't say it has to release it's prey. Mandatory actions are self contained and never require you to do other things to make them possible. So the only requirement is that it must move, which it can't do unless it voluntarily releases its prey. As long as it doesn't decide to do that, it can choose to do other actions. It is exactly like the Gremlin or Vampiress.
2. Siren's Call is very specific in that the Mandatory move has to bring you closer to the Siren. If you can not move closer, then you are free of the mandate and may do any other action you wish. In your scenario, there is no possible move that would bring you closer to the Siren. Going around the wall requires you to (first) move further away from her. Now, if you were to voluntarily move out of the zone, and are unhindered, then you'd be forced to spend your Quick Action to move closer to the Siren, but just like in question #1, that first move is not part of the Mandatory Action and so has to be something you voluntarily chose to do.
These are really good questions to ask. Knowing the rules on Mandatory Actions have saved my bacon over a dozen times with Chant of Rage, resulting in them basically wasting at least one round of the enchantment.