As to the first part of your question, i refer you to the excellent answer by @phantom42.
The other part of your question will be answered based on my knowledge of the movies and the script, as can be seen below.
Why he insisted
Actually, Panaka didn't insist; the Queen did.
Panaka states that it is the Queen that has commanded (and wished) the handmaiden be brought along, with him only acting as the messenger of this command. I think it's fair to assume that he was given these instructions off-screen, by the Queen (Padmé), and simply acted on them.
Part of the movie script, conveying the exchange between Panaka, Qui-Gon and Padmé
From the spaceship, CAPT. PANAKA and PADME run toward them.
CAPT. PANAKA Wait!
QUI-GON stops as they catch up. PADME is dresses in rough peasant's
CAPT. PANAKA Her Highness commands you to take her handmaiden with
you. She wishes for her to observe the local...
QUI-GON No more commands from Her Highness today, Captain. This
spaceport is not going to be pleasant...
CAPT. PANAKA The Queen wishes it. She is curious about this
PADME I've been trained in defense... I can take care of myself.
CAPT. PANAKA Don't make me go back and tell her you refuse.
QUI-GON I don't have time to argue. But this is not a good idea.
Stay close to me.
He gives PADME a stern look.