Theon's father decided to attack the north because they were distracted with things in the south. Theon did what he was supposed to, which was go and try to get help. His father was like lol fuck that, decided to call himself king of the iron islands and invade the north. He wasn't going to just attack random parts of the north and leave the seat of its power untouched. His son WAS still a ward. When Theon wrote out the letter, thinking of warning Robb, what would he have been writing to warn Robb about unless Balon had already made his decision to go forward with the plans? Perhaps Bran and Rickon wouldn't be thought dead...because they'd actually BE dead because Balon's men most likely wouldn't have gone in like Theon did and politely ask them to yield. I'm not sure where the idea that the castle wouldn't be almost empty comes from. Like I said, I thought the whole reason Balon decided to strike out was because the north was vulnerable being preoccupied with the south. Theon actually made things a little less bad for the northerners and worse for himself by attacking Winterfell. He tried to minimize casualties and spared the true lords. But now he's cockless, without any real family besides his sister, and likely going to be killed after Ramsay has his fun. If he didn't take Winterfell, he would have either stayed at the iron islands (doubt that for some reason) or went back to Robb. In which case he would have died at the wedding. Because Balon would still have attacked Winterfell, but with more men and better thought out, Robb would have still married Talisa, Roose would have still sent his bastard to take back Winterfell, Catlyn would have still released Jaime, the Karstarks would have still been pissed and lashed out, etc . The role Theon played in the destruction of Winterfell is quite small, the way I see it.
This is a very poorly structured paragraph/blob-o-text...Oops