Thanks!
Both 
hov:lanes:... and lanes:hov:... are valid tags. The first is an access tag with the :lanes suffix (i.e. which lanes are allowed for hov) while the second is a lanes tag for a special type of traffic (i.e. how many lanes for hov exist).

There's a page in the Wiki detailing the most common order in keys: https://wiki.openstreetmap.org/wiki/Order_of_key_parts