I feel your pain when it comes to the chewing the collar thing. We went through 6 or 7 with our Quincy til he was about 18 months old. Just when we thought he was over it and spent big money on a good leather one for him,,,, Nope,,, chewed that one too. They need to be tight, the two finger thing is a food rule to follow.
As for training look at my site, I have proven methods that you can read that may help.
Dawn's Yorkies / Yorkie Puppies / For Sale, Ohio