Stepping on brake peddle doesn't always allow shifter arm to move out of park

You may need to get it adjusted. Also, you can check to see that the brake lights are working each time you step on the brake. The system is usually controlled by the brake switch which is usually above the brake pedal. If you want to find a specialist, here's a directory link for you: https://repairpal.com/mercury-repair-in-racine-wisconsin