You're welcome. It sound s like you did a great job of helping your daughter. Concerning your questions:
Lack of vitamin C can cause many different symptoms, including paralysis. However, I have not heard of it causing a tilted head, dizziness, falling over, and turning in circles. I suspect something more was going on with fuzzy. If you get another guinea pig in the future, the best way to provide vitamin C is with lots of high vitamin C produce: sweet red peppers, strawberries, orange, tomatoes, etc. Adult guinea pigs need about one cup of produce per day. You'd want to introduce smaller amounts to a baby until it gets up to this amount.Introducing it too rapidly can lead to diarrhea. There is no vegetable or green that should be fed every day. Variety is very important. Cruciferous vegetables, such as cabbage, collards, bok choy, broccoli, kale, and Brussels sprouts tend to produce too much gas, and shouldn't be fed often. Onions and garlic are not good, either. Tomatoes, red peppers, summer squash, and greens (no iceberg lettuce) are all good. Clover, dandelion, plantain, and grass from an UNSPRAYED lawn are excellent choices. Guinea pigs can also have fruits - strawberries, oranges, and kiwi are enjoyed. there's a lot of misinformation available online. The Guinealynx site is one of the most reliable guinea pig sites. This link will take you to their list of recommended produce:http://www.guinealynx.info/diet.html#vegs
Some substances in plastic can cause cancer, so if Fuzzy had a brain tumor, that could be a factor. Plastic wouldn't normally result in outright poisoning. The most common problem that results from eating plastic is an intestinal blockage. Fuzzy had no symptoms of that.
The length of time vestibular disorders go on depends on the underlying cause. But it usually causes discomfort more than pain. The constant dizziness and inability to move properly is disorienting and confusing, but not painful.
If you need anything else, don't hesitate to ask.