Forget to add... my brother experienced the same codes on his Sienna minivan. He was getting a lot of smoke as well, but it wasn't sweet smelling. Plugs were fouled pretty badly like yours. His was caused by sludge and then bad rings. He was getting tons of blow-by. The sludge issue was clearly a manufacturing defect, but Toyota refused to pay for it because they claimed that in order to qualify he had to prove that he had service done every 3,000 miles (WTF? That's not even the recommended service interval in his manual!) and it had to be done and documented at a Toyota dealership. So they were denying claims if you didn't use their service, which is 100% against the Magnusson Moss act. Look it up and be prepared to fight if the dealership wants to be jerks.
I've been through a similar situation on a head gasket with Chrysler (what a mess!) and they had replaced it twice under warranty. As soon as the warranty was up it happened again. Of course it was due to their faulty initial repairs in warranty and I did everything I was supposed to. It happened again outside of warranty, which was THEIR fault. They refused to fix it. I brought up Magnusson Moss and their tune changed. I have no idea whether it was even covered under that act, but the fact that I knew about it seemed to scare them. They then claimed they would do a "one time good will repair" on it. I said that wasn't acceptable, and they'd do it until they got it right whether it took them once or 10 times. They'd already had two cracks at it and blown it. Luckily there was a redesigned part and I printed off the freaking TSB for them and made them use the correct and redesigned head gasket. There's a reason I don't drive Chryslers anymore. CRAP.
Don't back down. If you don't like what you hear right away then try another dealership or call corporate. Spend as much time on the phone as needed. Make sure they get it right.