Just why does the series have a pumped-up, Hot-Blooded opening theme contrasting with a soft rock ending? The final episode drives the point home - Yusuke, finally unburdened by the duty of being Kuuga, shows his kindness to a group of children with the ending theme playing over. The two musical themes of the show present the character of Yusuke Godai - a kind man forced by circumstances to become a fearsome warrior who would save the world.
The fact that three of the Grongi are based off Kamen Riders, namely Badaa, Gadoru, and Daguva, makes it practically foreshadowing for the fact that Kuuga's powers are no different from theirs.
The opening song's first few chords actually show off Kuuga's forms.
The first chord is pretty normal compared to the rest, resembling Mighty.
The second, however, is quicker, resembling Dragon.
The third is played in a different audio channel compared to the others, resembling Pegasus' Super Senses.
The fourth is bigger and louder than the others, resembling Titan.
The final chord before the lyrics hit is a big, devastating riff. This is pretty much what Ultimate Form is.
When Daguva was gleefully laughing during the final Good Old Fisticuffs battle, it suddenly became clear that N-Daguva-Zeba wasn't fighting to win. Kuuga was also a Gurongi himself (an immature one) and Yusuke was simply the human host for the transformation. The final fight was to truly bring out Kuuga as a Gurongi so he can usher in an eternity of violence. In short, Daguva was hoping that Yusuke would lose what's left of his humanity in that fight.
Why is it only the police, and not the military, who are engaged in fighting the Grongi?
Because they are more like serial killings. Declaring martial law at the city will make the situation worse than usual.
There's also the fact that the military likely wouldn't get involved unless the Grongi spread their activities over a wider area - as far as they're concerned, as long as it's not too widespread, it's no different from a serial killer on the loose (something the police would be in charge of handling).