Let's see... the first question was:
"What is the real scoop on formal nights for dinner? Would a dark sports coat, slacks, and tie do the trick, or does it have to be a tux or dark suit? Thanks."
A polite question at that.
The real scoop is from Carnival FAQ's:
http://www.carnival.com/CMS/FAQs/Dining_Room_Dress_Requirements.aspx
"Is there a dress code for the dining room?
Depending on the length of the cruise, either formal dress or casual resort wear is required in the Dining Room. The dress requirement for children is the same. For those not wanting to dress up, the Lido restaurants are open nightly (with the exception of the last evening of the cruise), and have a casual dress code. Formal Wear: tuxedo (optional); suit and tie; sport coat, tie and slacks; evening gown; cocktail dress; pantsuit Casual Resort Wear: sport shirts and slacks; dresses; skirts; pantsuit NOTE: Shorts, t-shirts and bathing attire are not permitted in the Dining Room during dinner. "
I just asked how to get my teenager to comply with the rules. The answer to that was simple:
"During last year's cruise on the Inspiration, we insisted that our 15 year old dress up in a button down shirt, tie and slacks for formal nights. He hated it...right up to the point where the teen girls on the cruise told him he was HANDSOME. He kept the tie on the entire night on both formal nights and this year he's asked us to buy him new dress clothes, including a nice sport coat.

"
Weather he agrees or not, he will comply. With us it's not the cost, it's a teenager's attitude. If he goes to the formal, he will wear a tie.
Now, let's not start a fight, that's what God gave us children for!
I will be formal, suit that is, not a tux. My son will wear a tie. With or without it, I'm sure the girls on the boat will still adore him. He doesn't really see what he wears, I don't see what I wear. It is everyone else in the room that see us. We don't dress up for ourselves we dress up for others.
Tom