Dental issues that can be corrected do not involve root growth, Once the roots start growing down into the jaw or up into the Jaw that is sadly not corectable and you delay the inevitable
This is not true. If the root is positioned where it can be removed without affecting other systems, removing it (and possibly part of jaw bone as quite often the only way to remove it is from the outside) corrects the problem. The consequent problem is that the teeth will not grow - and wear down - at the same rate since the rootless tooth will stop growing but this means you'll have to get the overgrowing tooth filed regularly.
I'm not saying this is possible in every case (and I would imagine it would be very hard to do if it's growing upwards) but it's not true that all chins with overgrowing roots are condemned to death.
Vit C - I know you've mentioned chewable vit c tablets but have you tried standard hard tablets? I supplemented Fluffy when she was pregnant and she just took it off me and ate it. I know that lots of people also say that fresh veg is not a great idea for chins because of bloat, however, the newest research suggests this is usually caused by lack of fibre rather than fresh veg. My chins actually get fresh spinach or kale simply because when i got my first one, my vet gave me a list of things to feed them. I have six and five of them get fresh veg (only got the sixth one a week ago and she needs a lot of tlc and not too many change in diet at the moment) and i've never had a problem. they only get a tiny bit like, 2-3 leaves.
I recently stumbled across this website
http://www.chinchillas4life.co.uk/diet.html - you may find that interesting as it describes a died of dried veg and herbs adapted from wild chin diet that's helped their chin who had teeth problems.
One more thing - pellets are fortified with vit c, however, it goes within a couple of months after opening the packet so if your pack lasts for longer than that, you could get smaller size or chuck it and open a new one.
Good luck! I just got a rescue chin a week ago that has a huge overgrowing root. At the moment, she needs to put weight on before we can even do an xray and she was fed so much rubbish before she came to us that she's not sure what it is she should be eating. But I really do hope that we can help her. xx