It's called a "construction impersonnelle", like "il pleut" and "il neige". Those are always used with "il", never another subject. More examples here: http://fr.syvum.com/cgi/online/serve.cgi/grammaire/verbe/verbes_impersonnels.html

It's closer to English structure. You'd say "it's 8 o'clock" not "he is 8 o'clock" so in the same way, it's "il est 8 heures" and therefore the question becomes "il est" inverted - "quelle heure est-il ?" It's the same for the temperature. We use the verb "faire". Ex : Il fait 30 degrés. The question would be "quel temps fait-il ?"

'il' isn't representing 'heure;' it just means 'it.' You could think of 'il' representing 'maintenant' etc. :p

I think il is just a pronoun here. Like in English It's John who runs faster than me.