I have no exact knowledge about the answers to the questions you posed, but I will try my best.
1. I have no idea, but you could probably test it. The traditional non-scientific test for determining metallic composition is to touch the piece to a grinding wheel in a relatively dark area and observe the sparks. This is an acquired skill among experienced machinists. You can learn a bit on YouTube if you are interested (How to Identify Metals Using Spark Testing - YouTube, ex.). My grandfather was a tool and die maker and had uncanny proficiency with this technique. Everything got touched on the grinder before it was machined. Brass does not to my knowledge come in numbered alloy grades like steel does. Certain other alloys of copper do not spark at all and are used for tools in explosive environments.
2. All copper is reactive with the environment over time. The Statue of Liberty was bright metal in 1886 and took years in a salty environment to achieve its current patina. I have a cornet with pinhole leaks where saliva has (over decades) eaten through the metal. Copper plumbing pipes suffer from this same problem and and have their own alloy for this reason. Maynard Ferguson's trumpet is uncoated brass and exhibits a patina. Virtually all other brass instruments are plated on the outside. Lacquer is most common because it is cheap and durable. Silver plate is also common on brass instruments but tends to wear through, especially when thin. Source for tools and chemicals commonly used for brass instruments: https://www.ferreestoolsinc.com/
