let's say I wanted to learn the grammar of/how to parse a context-sensitive grammar from a 100 or thousands of valid examples -- what's the best approach?
Currently recurrent neural networks are the best language processing ML algorithms. Here is a relatively new paper on recurrent neural network grammars and it references many of the recent papers:
Are you sure it's context sensitive grammar rather than a collection of N context free grammars? Many file format standards can be expressed as context free grammars.
If you are trying to figure out something like "what is the unknown XML structure" based on an example file or files then you are firmly inside natural language processing.