Rolling window features for multiclass classification
Usually the point of a rolling window is to calculate and use a moving average:

A moving average is commonly used with time series data to smooth out short-term fluctuations and highlight longer-term trends or cycles.

By definition the only meaningful aggregation for this kind of purpose is the mean: the idea is to represent the same type of value as the original series while correcting noisy variations.
Anything else is not the standard use case for a rolling window, so the answer to questions 2 and 3 would completely depend on the application.