Wut? You can sue Maddow directly for slander. The main reason you sue the employer is for the bigger pockets.
This - reporters don't have any sort of immunity that would shield them from a lawsuit.
That said, this would be a slam dunk win for Maddow - they were reporting Russian propaganda about the Syrian chemical attacks, and Rouz is paid by the Russian government. The quote again (all caps from the link):
Maddow said:“REALLY LITERALLY IS PAID RUSSIAN PROPAGANDA.”
“THEIR ON-AIR U.S. POLITICS REPORTER IS PAID BY THE RUSSIAN GOVERNMENT TO PRODUCE PROPAGANDA FOR THAT GOVERNMENT.”
For OANN to win they'd have to make the case that she meant they were exclusively Russian propaganda, which is a pretty tortured reading of her statement. The weakness of their case is evident in the complaint - where they're stating that OANN is owned by Americans, and that Rouz isn't an employee of Sputnik, but merely a freelancer. But she never stated they were owned by Russians or that Rouz was an employee of Sputnik.
no, reporters do not have any "special" protections of that sort that I know of... she is an employee doing her job, like everyone else. As long as her employer cannot substantiate that she was operating with malice, complete irresponsibility, or some other drastic departure from her expected work behavior that could be called "gross misconduct" from the DoL, then her "protection" is her status as an "employee". The company is liable for their employee's actions unless they can claim she went totally rouge.