There are substitutes for certain specialties. For example, there are several procedures in gastroenterology that are performed by general practitioners who could be alternate providers in case merged gastroenterology practices might have raised their price. Another tool that's available is to ask the Department of Justice or the FTC for an opinion, which they will provide at no cost. I think it takes anywhere from eight months to a year or so, depending on the complexity involved and the department's work load. There's a body of case law on both sides of the issue.
Some plaintiffs try to use peer review to uncover incompetence.
Some defendants try to use peer review to derail competitors. The Health Care Quality Improvement Act of 1986 is what grants the review immunity at the federal level. There are state immunities also. The plaintiff would allege the peer review procedures had been breached and that therefore peer review immunity didn't attach. Under antitrust laws the plaintiff could allege attempted monopolization or conspiracy to restrain trade under the Sherman Act.