Questions about Automatic Weather Updates - Xplane

After using FSRealWX a couple of times with XPlane I have some questions about how the Weather is updated:

In Options there is a "Weather Update Interval" (default 30 min). This is how frequently new weather updates are downloaded from the server. So before this interval expires, I guess FSRealWX holds the same cached copy of the WX from the last time WX was downloaded.

For XPlane there are additional settings:
Xplane - Weather update after (only XPlane)
Distance 20NM, Time 5 Min

I am guessing this means FSRealWX sends its weather data to the sim every 20NM or 5 min (whichever comes first?) on the condition that the current ALT is above the Alt for updates (default 5000ft).

Q1) So if ground speed is any faster than 4NM/min (GS > 240 KTS) the 20NM distance threshold triggers an update, otherwise the 5 min time triggers the update?

If the Weather Update Interval is set too high (e.g. 30 min) then every 20NM/5min the same cached weather would be getting sent to Xplane by FSRealWX.

Q2) In this case, if the reported weather is changing quickly, should the Weather Update Interval be set to something less than 5 min?

Finally, assuming the automated updates work as described above:

Q3) When/why would I need to use the "Set all WX" or "Set ICAO WX" buttons and what is the difference between them?