1) Overreacting is a question of how valueable your protected information is. My personal oppinion is that by the nature of digital computers and of the internet there is no such thing as overreacting in security. You either want something to be public or you do not, and in the latter case there is quite a bit of work and knowledge involved to counter all the different attacks. On the other hand, a lot of other people will say that if your protected information is not really valueable enough to hackers, then probably no hardcore hacker will invest enough time so your protection might not worth your programming efforts.
2) In the code above you build your fingerprint by hashing a concatenation of different information about the remote user and the token is used as a kind of salt (in addition to 'SECRET-SALT') to make that hash unique. So the answer depends on how secure is your session storage.
If you are alone on the server or you have a secure session storage for $_SESSION (like a password-protected database), then you do not need to worry about someone reading the contents of your session variables. This also means that protecting your fingerprint by hashing it is absolutely useless, in which case you can just remove the token and hashing from your code completely. In this case you also do not need a fingerprint, you can just store the user variables separately and compare them separately.
If there is a way for your $_SESSION contents to leak, then you want to protect private information in it. In this case you will want to keep your token private and do not send it to the user, put it into login.php instead. Normally, you do not have to worry about protecting a salt because even if it is known it will increase the time considerably for a hacker to crack a password, not just because of the longer password string but also because precomputed rainbow tables become useless. But in your case you do not have a password, all you have is the user_agent and the IP, both of which are easily predictable or sniffable. This means that the concatenated information is basically no variable to the hacker, so the only thing keeping him from cracking your fingerprint are your salts. So if you reveal them, the hash becomes useless.
To sum it up: If you want maximum securty in all situations, keep fingerprinting and hashing, but since your protected string is easily guessable, keep your token private this time. Theoretically even if your token is compromised 'SECRET-SALT' will still provide protection, but only as long as your code is safe. If your filesystem is hacked or the sources for the website your are building are public you cannot rely on 'SECRET-SALT' alone. Do not depend on hiding it, 'security by obscurity' is a bad practice.
3) Generally you should not depend on the IP to stay consistent across a session. This is because for a lot of legitimate users the IP can keep changing constantly, this can be because they are behind corporate load balancing, they are using AOL or some other proxy network. A possible solution is to observe the IP of a session and if it is consistent for, say, 10 pages, then it is reasonable to assume that it will stay consistent in the future. So only then will you start making checks on it. However, storing it temporarily in a DB can still be of use, eg. for logging so that it can be analyzed what happened if there is an attack or so.