Throwing the head around to try and get the food into position to swallow it, is classic dental! I truly believe there is still a dental issue here and until that is resolved your piggy isn't going to be able to eat. I have been running the dental sanctuary since 2011 and have seen so many piggies with dental disease and time and time again the guinea pigs have had unsuccessful dentals elsewhere and have been eating before they've even left the practice, after a dental by Simon Maddock. Simon can also carry out dental work without GA, so there isn't an anaesthetic for the piggy to get over.
We currently have a temporary resident from Devon, who arrived after having a dental, bloods taken and x-rays, amounting to well over £500, at a very well respected veterinary practice, but still he couldn't eat. After Simon filed his teeth, he was eating immediately.