You can test the ammeter by connecting it in series with any load, such as headlamps. Just connect it and check it before you put the 4 screws back in to hold the panel in place. My original ammeter is quite accurate and draws about 8 amps to run the lights.
Would me disconnecting the ammeter isolate the alternator and explain why the car runs fine now? All the accessories work when the car is running, they just don't with the alternator not turning. I can easily pull the alternator and take it to a local shop to have it checked out.