I don't see an answer to your question about the meaning or use of "STAT$RAIN:CURRENT$". It is a confusing construction to me. I thought "STAT$" was used to get 'historical info from the WC database. In this case you seem to want the 'current' rain amount. I would suggest simply using the RS$ tag would give you the "Current rain storm value" and the user set units; "6.0 mm".
OTOH, "CR$" might be what you wanted, that is the value of the "hourly precip
rate"; " 0.8 mm/h (0.1)" means .8 mm current rate, as compared to the previous rate of .1 mm/hr.
Instead of "STAT$TEMPERATURE:CURRENT$" I think all you need is CT$. I think all tags starting with "C" provide 'current data'. Likewise, "CW$" provides the average current speed in place of STAT$AVERAGEWINDSPEED:CURRENT$
At least WC doesn't issue any errors, but it looks counter-intuitive to see "STAT$" and "CURRENT$" in the same tag combination.
I'm not sure you understand that "current" values will be generated every number of minutes the user defines in the WC Prefs. Mine are generated every 2 minutes, which is probably fairly common. I think you'd need to have the user enter this frequency in your setup GUI so you'll know how often to check his uploaded data file. You could use the CU$ tag and check for changes every 30 seconds for several minutes and see when that value changes, but that seems like a pretty brute force way to do it!