An in-house nurse isn't going to be put on the stand as a testifying expert, because as an employee of the attorney you would then have a vested interest in the success of the case and you would be shot down for bias. The attorney, meanwhile, has hired you to be the in-house nursing expert to help figure out whether a case has merit from a medical/nursing standpoint. There's no way an inexperienced nurse can take a 600-page record (maybe several of them a week) and get through it efficiently to get the gist of the medical issues in the time frame needed. The attorney will rely on you to give them the good news and the bad news, because if you can find bad news, you can bet the opposing expert can find it.
If you are a legal nurse consultant who might be asked to testify, you will be testifying about your area of (current) expertise. You are hired by either the plaintiff or defense to give her best information, because the role of a testifying expert is impartial; that is, even though the hiring attorney is paying you, you cannot slant your testimony. You are not a patient advocate in this role. The attorney is the advocate. You are responsible to the court to give your best estimate of the truth, without reservation. If you don't do this and your testimony is discredited, you lose a lot-- you can't be trusted, and you'll have a hard time getting more work. You have to turn down cases you're not qualified for, because if you start faking it, you will be found out.
So there are two different roles here...and neither one is going to be offered you without a convincing set of expertise, analytical ability, speaking/writing/teaching ability, and credibility. All the nurses I know who are working in these roles are really, really not rookies.
If you're interested, go to the AALNC, American Assoc of Legal Nurse Consultants, for info on courses and certification. Another choice would be the American Assoc of Nurse Life Care Planners, AANLCP, for life care planning, which can be a great consulting gig.