Nitpick: You're right that the C# designers don't have to worry about a JCP-like standardisation process, but they do put some effort into producing a formal language standard (I think through ECMA). Unlike earlier Microsoft proprietary languages, where the language definition was whatever the compiler said it was, the C# team do try to pin it down, make sure all the edge cases are considered, and so on. The compiler implementers try to write to this spec. They don't always succeed, but from what I read on compiler guy Eric Lippert's blog, when they do diverge, he regards that as a bug in the compiler, not an error in the spec.
That said, the standard is very much owned and written by Microsoft. You're absolutely right that they don't have to work through a JCP-like process.
Yes, you are right. Despite it's proprietariness, C# enjoys having team that is very rigorous and have a great deal of compiler experience. Eric Lippert is super cool. In fact, C# remains one of the reasons why I still root for MS.
That said, the standard is very much owned and written by Microsoft. You're absolutely right that they don't have to work through a JCP-like process.