(b) That's probably right. It is most unlikely that the courts would issue an injunction in these circumstances to prevent A from employing a new manager. Why? Because it is not possible for A to work as a model without a manager, and if she is prohibited from employing anyone except B Co., the result is the same as specifically enforcing a contract for personal services. A's only alternative is to stop working as a model and she has no other means of earning a living. The courts would not issue an injunction in these circumstances.
Remember, an injunction is an equitable remedy. This means that a court always has a discretion whether or not to issue an injunction, and will not do so when the result would be harsh or unfair.