It has been a very long time since I was in that situation, and my situation wasn't exactly the same (I was married but I was working in a status that provided no derivative status for a spouse), but I can give you my understanding of the issue.
US immigration law does not recognize a relationship between unmarried, but de facto, partners as a qualification for a derivative (or any) visa. The B-2 visa is sometimes suggested for de facto partners not because it is an appropriate status for a de facto partner to join the visa holder but because it is the only status available for her to enter in, appropriate or not. It is a miserable, uncertain way to stay since her continued presence in the country and the conditions of her stay depend on the discretion of every officer she sees, the officer's willingness to recognize the validity of the relationship within the constraints of the law they enforce, and whatever policies-de-jour they are operating under. Note that your experience at the consulate, with them granting a visa (I imagine) you applied for with the expressed purpose of her living with you while simultaneously telling you it wasn't (legally) suitable for that purpose, is probably a typical example of the ambiguity and uncertainty you'll always be dealing with.
For the questions, there is no guarantee that they'll let her reenter but, since there is no formal link between her status in the US and your's, you'll maximize the probability of success if you always accompany her so that the situation is clear to the officer whose discretion you are relying on. Stays of up to a year are possible but I really don't know if you should explicitly ask for that or just try to suggest you'd like her to stay as long as she can (from the comment above it seems like you should); if you want to stay in the US longer than they give her you can always apply for an extension of stay while remaining in the US. And, yes, they can refuse her entry, on the current visa or a new one; there is no guarantee. They will be relying on your status, the fact that you are a non-immigrant and will eventually be leaving for home (with her) after a longer stay, to conclude that her own long stay does not imply immigrant intent and is hence permissible, but you are at the mercy of the discretion of each officer you deal with.
Whatever you do she should be scrupulous about maintaining her status and conditions of stay. The first sign that she is anything but compliant will likely end their tolerance.
It used to be that stays on this basis were routinely granted (and my wife got 1 year stays, though that was long ago in somewhat different circumstances), and the fact that she got the visa is a hopeful sign, but it was never without uncertainty and I have no knowledge of how they treat this currently. There is no legal basis that would allow her to expect to be admitted, you can only hope they do. If you want greater comfort you could try speaking to a lawyer who has recent experience with this.
