Couple of thoughts here:
Some of your schema questions could be answered by simply following WMO standards. Like weather observations are typically reported in UTC. Wind speeds are reported in KTS (I know everyone thinks that because the U.S has not converted but really it’s the WMO standard). Temps in Celsius. So on and so forth.
Secondly, (and by applying this statement I’m not proporting favoritism to the U.S NWS) why not apply a schema similar to the U.S NWS XML “standard”. Or better yet, determine the national schemas and utilize them. I’ve utilized the NWS so that when my personal weather station is off line, I can easily use the closest NWS site because the schema doesn’t change. Also, if all of the local personal weather stations did similiarly a “standard” XSL could be applied to all obs utilized by a website for use in ‘plotting’ obs.
Just my thoughts.
Paul