Let me first clear up a little mistake you made. There was no jury at Nuremberg, only a panel of judges - one judge and one alternate from the USSR, USA, the UK, and France. All eight ended up having the same power, participating in the decision on who to convict and what to sentence them to.

I'll answer the side question first. At the IMT (the trial of the major war criminals by the International Military Tribunal), all of the lawyers were Germans - the article appears to be inaccurate there. At the IMTFE (International Military Tribunal for the Far East, the Tokyo trial of the main Japanese war criminals) many US lawyers were involved in the defense and two participated in the NMTs (the subsequent Nuremberg trials, run by the USA only). Here is an older answer of mine about why they took part there.

Now, as for question 1. Some of these lawyers had been Nazis, others hadn't. The German bar had been very heavily Nazified, so barring anyone who had been in the Party would have been simply impractical. One of the judges said that "it is all right to let a member of the Nazi Party appear, but it is probably not wise for the Court itself to select one to represent the defendants" (Tusa 123), and the others agreed. Some of the defendants asked their relatives or family lawyers for advice. The tribunal encouraged lawyers to volunteer for the task, making repeated assurances that the defense of Nazis would not be held against them - in fact, when the press criticized the lawyers for defending Nazis, the tribunal stood up for them. Dr. Robert Servatius came forward and ended up defending Fritz Sauckel. Sixteen years later, Servatius went through a Mossad background check before he was allowed to become Adolf Eichmann's defense lawyer - clearly, his past was acceptable.

Some lawyers were sincerely glad to defend their client, or Nazism in general. Others considered it their duty to defend their client, no matter how blatant their crimes or how obnoxious they were. Poor Dr. Merkel was horrified when he got a summons telling him he was to defend the Gestapo (Nazi organizations were also put on trial, similar to how political parties or similar groups can be banned) - he had no desire to defend "those hangmen" and had to be prodded into it by his brother-in-law, who reminded him that a lawyer has to be impartial (Tusa 124). The main reason why non-Nazis were willing to defend Nazis was that being a defense lawyer at Nuremberg meant plenty of food, good pay, and a roof over one's head. In the shattered cities of postwar Germany, that was a very powerful motivator. After the trial, most of them had perfectly unremarkable careers. A few wrote articles about the trial, those with imprisoned clients tried to get them released. Alfred Seidl, the lawyer of Rudolf Hess, spent forty years campaigning to have his client freed from prison - he failed.

It is interesting that you mention Doenitz, because his defense was probably one of the best. As soon as Doenitz found out he would be allowed to pick a lawyer, he asked for Dr. Otto Kranzbuehler, a naval officer and lawyer who was surprised to find out that Doenitz was being indicted at all. He was eager to defend Doenitz and the German navy by extension. One of the main charges against Doenitz was that of unrestricted submarine warfare - the sinking of ships without warning - and that his submarine crews had killed survivors. Kranzbuehler got Chester Nimitz (US admiral) to write an affidavit explaining that the USA had also used unrestricted submarine warfare. It wasn't that they had both broken the rules, it was that the rules (written before WW1) were simply not applicable anymore and there was no point in judging people by these standards (for example, merchant ships were now armed and could use the radio to call in backup, so they weren't exactly defenseless). Basically, by the standards of US practice, Doenitz did nothing wrong in his submarine warfare. The judges were split on it - most thought he should still be convicted, others thought it would make no sense. When it came to the killing of survivors, Doenitz' order to not help survivors was deemed to be not a kill order but simply one to not rescue survivors - still criminally ambiguous, the judges were split on whether he was criminally liable for that. What Doenitz could not evade was the fact that slave labourers and concentration camp inmates had been used to build ships. He had also been a fanatical Nazi, saying things like "the spreading poison of Jewry" in speeches, but he wasn't on trial for being antisemitic. There were also things like his handling of the 'commando order' (order to kill captured commandos on the spot even if they were in uniform). The judgement against him is impossible to understand, as it was written by the one judge who wanted to fully acquit him. Doenitz was found guilty on Count Two, waging aggressive war, because he had been "of unusual importance to the German war effort" (Smith 263). The attacks on armed merchant ships were ignored and attacks on neutrals condemned. When it came to Count Three, war crimes, Judge Biddle (who wrote the judgement on him) stated that he wasn't being assessed for his breaches of the international law of submarine warfare, so he was condemned for relatively lesser things like his handling of the commando order and his suggestion to use concentration camp labour for ship building. Basically, the majority of the judges decided to convict him for submarine warfare but agreed to say it was on other grounds. Doenitz got ten years in prison, served it to the day, and lived until 1980, convinced he had done nothing wrong to the end.

The case against Goering, by comparison, was simplicity itself. His lawyer, Dr. Otto Stahmer, was convinced of Goering's innocence, but there were piles and piles of incriminating evidence against him. During his direct examination, Goering did his best to defend the Nazi system. He talked with pride about rearmament and "winning living space". But by the end of the cross-examinations even he was weakly insisting that he hadn't known about this or that - claiming ignorance of atrocities, especially of the genocide of the Jews, was the most popular excuse at Nuremberg. All eight judges agreed to find him guilty on all counts and sentence him to death.

If you have followup questions, feel free to ask them :)

Works cited:

Reaching Judgement at Nuremberg by Bradley Smith

The Nuremberg Trial by Ann and John Tusa