I'm not a lawyer, but Groklaw answers a fair amount of this. I've also sat on a jury before, if that makes any difference.
1. This is one of the points being argued. The gist of Samsung's arguments is that there is a legal standard that believes that a prospective juror under oath is to be believed unless there is reason not to believe. The threshold for breaching a prospective juror's privacy is much higher than that for breaching an interested party's. Jury selection is long and complicated as it is. When a juror says, "I was involved in 1 lawsuit involving XYZ" and there is no apparent need for follow-up on other suits, lawyers typically won't follow up. There is trust that jurors will be forthcoming, because they took an oath that they would be forthcoming.
2. "Protected" is a complicated word. Basically, the Court issues instructions to the jury, and trusts that the jury will abide by those instructions. It requires an extraordinary level to prove that a jury acted outside the bounds of the Court's instructions. It's one thing if the jury's verdict doesn't jive with what the Court thinks it should be. It is another matter entirely when there is evidence of willful misconduct by a juror. Basically, if it can be demonstrated that a juror was willfully disregarding jury instructions or otherwise was acting as an "interested party", that juror could face sanction from the court, including the possibility of having to pay at least some of the costs. It essentially comes down to jury tampering. The bar for proving this is very high, but a juror's own words after the trial can be used against him or her.
3. This is also a complicated question. Lawyers want to win, yes, but they also have a fairly rigorous set of legal ethics to which they must adhere. This is a civil trial, so they are not under the same burden a criminal prosecutor is. Apple doesn't need to make Samsung's case for Samsung. At the same time, anything they plan to introduce at trial needs to pass through Samsung first so that Samsung may object or present a defense. Cases like this have very few "Aha!" moments. They have TONS of filings, briefs, depositions, cross-depositions and so forth. The court's job is to make sure the trial is fair and that both sides get their say. That said, if Apple had prior knowledge of juror bias, they did have a legal obligation to make the Court aware of this bias.