Im so sorry for your loss
Did you fully read the contract you signed to have him groomed? Its so important to read what you are signing, it could of said something about a seditive in it and it should of stated the risks that come along with the seditive. Some little dogs just cant handle those things, there could of been underlying cause of why he didnt tolerate this medicine. One should always do bloodwork on your dog before you give them any kind of seditive, to make sure everything is working right inside. A popular seditive Acepromezine some little yorkies cant handle and can make their heartrate drop down to almost nothing.
I would personally have a different vet do the necropsy, one who isnt connected with his passing.
I see alot of people posting bad things about groomers and some of the stuff I read is shocking. I can never imagine treating someones dog so bad. Just yesterday alone I had to yorkie people tell me that they just cant seem to find a good yorkie groomer.