I don't buy the cheating allegations for the simple fact that no one has ever mentioned it at all until now. Some of the chefs have been asked numerous times why they didn't like Marcel and to give specific examples of bad behavior, but the worst they could say is that he's annoying or he uses an "insultory" tone when he talks to people. If he had cheated in the middle of the competition as Sam is now saying, someone would've mentioned it by now.
As far as the agar goes, they sell it at Wild Oats. So if Elia saw him using it during a competition that doesn't mean he cheated. He could've bought it fair and square with money that was given to him during the competition. When she says "I don't know" before the accusation -- that's a tell. She's not 100% sure that he cheated. She's just grasping at straws.
Finally, the TV.com poll wasn't hacked. Marcel is extremely popular on the Television Without Pity forums. After the assualt his fans rallied. They spent a whole weekend voting for Marcel until his votes passed the 100K mark. They rallied again after Elia accused him of hacking. Anyone can vote on that poll an infinite number of times. At one point Sam's numbers started jumping as rapidly as Marcel's. I think Elia's just pissed because she only got a handful of votes.