It is pronounced sort of like "hair" in English, but more quickly.
Is "hr." pronounced "herre," or is it pronounced in some other way?
Hr. Is pronounved "her" with it sounding like the "her" in "herre".
Why isn't Herre allowed instead of hr. ?
It can only be written as "hr."
"herre" means gentleman, master or lord.
If you want to write the full title, it's "Herr" without the 'e' at the end, when used as a honorific before a name.
"Herre" with an ending 'e' means a gentleman and is used e.g. in terms like "Mine damer og herrer" ('my ladies and gentlemen').