Question regarding debt collector in Michigan

10 months ago my dog was taken into an animal hospital due to low red blood cell count but otherwise completely healthy 11 year old dog. 48 Hours later he was dead. I paid a $1k deposit when first checking into the place and that is all I have paid thus far. The animal hospital never contacted me to collect and I figured it was because they acknowledge some responsibility for the dogs death, which I believe they are atleast somewhat responsible. Regardless, I got a letter from a collection agency demanding the remaining $4k payment. I may or may night hire legal help but I am first wondering some general questions:
1. At what point can a debt collection agency put a claim on my credit report? I have excellent credit and do not want to ruin it, but I cannot imagine that any collection agency can just ruin someone's credit but claiming they are owed money without having to prove it through some sort of legal process?
2. The letter said 30 days to pay up. I responded with a certified letter requesting proof of charges and their right to collect on behalf of the animal hospital. I have yet to hear back from them and received a second warning letter but not sure what to do?

Any help or insight is greatly appreciated