Jury nullification is not illegal in the United States. Judges and prosecutors do not like jury nullification, but a judge should know the difference between strictly legal and something they'd prefer to avoid.
A former juror being allowed to testify during a current preceding would be so grossly prejudicial that the defense would be within in its rights to demand a mistrial. Since the jury was formerly part of the body tasked to decide a verdict, seeing one of their former group on the stand testifying against the defendant in the same trial would more than enough to have the verdict overturned upon appeal.
When Chef Moretti is first questioned by Detectives Joe Fontana and Ed Green in his restaurant they ask about a band aid he has on his thumb. He explains that it was a chef's accident. However, the band aid is light flesh coloured - chefs and kitchen staff must always use commercial blue ones as these are special made for food handlers. They are waterproof or water resistant, have strong adhesive so they are less likely to fall off, and because they are bright blue in colour they are easily spotted if one should fall into food.