What are the criteria and definitions of mean wind speed, sustained wind speed, and maximum wind gust?

Unfortunately, there is no straight-forward answer.

The World Meteorlogical Organization (WMO) sets the recommended standards for meteorological measurements, but it is quite possible that the meteorological services of individual countries may not have abided by those standards in the past and may not even do so currently. The WMO standards are available only in printed form and I do not have access to that publication. You should contact them for that information.

The wind measurement standards used by the U.S. National Weather Service (NWS) are given below, but they may be somwhat different from the standards of the WMO and other coutries.

U.S. NWS standards:

The NWS defines a wind gust as "the maximum instantaneous speed in knots in the past 10 minutes ..." (when there is a rapid fluctuation "in wind speed with a variation of 10 knots or more between peaks and lulls.") And wind speed is defined as the "2-minute average speed in knots ..."

-- Michael Bell