Thoughts and suggestions...
It could be that someone tried to discipline him by taking him by the collar or cheeks and staring him down or shaking him. Always a very bad idea, because it frequently evokes this kind of behavior later down the road with other people in less challenging enviroments.
Or it could be that his ears or some part of his face hurts. Many dogs do not like their ears rubbed, especially if they had previous or recurring infections. A bad tooth or prior neck injury could do the same. Did the vet check his ears, neck, teeth and general head area for sensitive spots? Did Toby seem to mind the vet's attention to those areas?
You mentioned that you had to be aware of where each of them is at all times. (I am assuming she lives in your home) Does he make unprovoked attacks on your daughter or change his posture when she's present (other than what you already mentioned)? Or are you just concerned that he might?
Your consternation may be adding to the problem. He may feel your worry and be constantly tense himself, especially if you ramp it up when your daughter comes in the room or is near. It would be very normal for you to become alarmed if she or he appears in the same room together, but you MUST control you're own emotions or you will not be able to help either of them. He may associate her presence with your worry and assume she is the cause, putting him on alert to some imagined threat. Has he ever growled or threatened her when you and she embrace, or are in very close proximity to one another? Does he ever try to get in between the two of you?
Your relationship with your daughter should always appear to be happy and relaxed when he is present. If you need to have an argumentative or serious mother/daughter talk with her, go in the bedroom and shut the door, or casually turn him outside.
I would have your daughter hand feed him every bit of his meal. She should do this in a non-confrontational way. Standing and not looking directly in his eyes, but just to the side of his face. He should remain in her peripheral vision at all times without furtive sideways glances on her part. If he's the nippy, grabby type, she should keep the food (treats, etc.) in a closed fist and only open it if he licks her hand. She can put a word with this action, such as "nice" or "gently" and not use the word "no" if he makes a simple mistake. Do not hold treats out by the tips of the fingers and allow him to snatch them away. He must be gentle at all times. She should praise everytime he does it right.
She should be the one rewarding him, not you. This will prevent you from rewarding tense, ambivalent behavior that you might not notice. Will he do simple commands for her? I would have her take over the obedience training with him (including some heel work, fronts and finishes) at home where he's more likely to pay attention. It should be upbeat and almost playful, not authoritarian. Her voice should be lilting, but not pleading with lots and lots of praise. She should understand and ALWAYS use a release word or gesture AFTER she praises him. It might be prudent to have a positive reinforcement trainer or someone else come to your house to give her some instruction. If you or anyone gives her instruction, please do so in a manner where you're offering suggestions ("try it this way") as opposed to "no, you're doing it all wrong". You're approach to both of them should be positive and encouraging, not critical. Keep the training sessions short and successful, ending on a good note. Don't push either of them past the point where they tire or become frustrated.
She should never, ever attempt to discipline him. If the behaviorist says he's a fear biter, then something she does is evoking that emotion in him. Is she afraid of him? Pop and jerk training is definitely not appropriate here. If he makes a mistake, simply start over or go back a step or two where he can be successful. Use what skills he has mastered as your starting point, but don't hesitate to go back to the very basics in order for him to earn a reward. This is all about building an amiable relationship, rather than teaching a dog how to do an exercise.
She could be playing fetch or hide and seek with him. If you're presence is a distraction (or evokes a protective response) and you feel she is safe alone with him, let them have some fun time together (you can watch from a distance or through the window). She should stay off the ground and remain standing, but her posture should not be tense. She should remember to praise him happily (Good boy, brilliant!), but refrain from becoming overly animated. She should not run, so as not to turn on his prey drive, but she could be somewhat bouncy in her praise and release him by lifting her arms, palms upward and saying "all done!".
Lack of bite inhibition is a separate issue. How old is Toby? Does he have any doggie playmates? Usually, dogs are better than humans at teaching bite inhibition, but if he doesn't get along with other dogs, then you will have to work on feeding him everything by hand and work on "take", "hold" and "give" for a time .
Having said all of this, your daughter must be a willing participant. If she doesn't have the time or isn't enthused, you may have to resort to life long management of the issues, instead. But if you are both committed to solving these problems, I think it is do-able with a lot of patience.
