Akamai = kēlā mākaʻi. That police officer is smart. That is a complete thought because the intent of the sentence is to describe the person as smart. Akamai acts as a verb here.
kēlā mākaʻi akamai -- that smart police officer - not a complete thought because you need a verb.
Not necessarily. You can turn this into a sentence starting with He, but there is nothing wrong with the original sentence at all. This type of sentence pattern is the origin of the same sentence pattern in Hawaiian pidgin -
Expensive that dress.
Pretty your shoes.
Smart that cop.
Your sentence with He should be "He māka'i akamai kēlā." meaning "That is a smart cop." as opposed to "That cop is smart." It gives the exact same info but just with slightly different words and word order.