As she was caring (supposedly) for the animal in your stead, she had apparent authority. The real question is, why was AC there at all? Someone...

Document her diversion and sue accordingly.

Again, retain counsel. You will not resolve this issue here.

You have both the dogs and the records for it. Ignore your ex's almost-certainly empty threats. In the highly unlikely event he actually sues...

"I don’t see my downpayment money applied to the loan of my car" So why did you not ask when you reviewed and signed the purchase documents?

Call the police and animal control - IF you can prove she did it, she is clearly irresponsible and a threat to your pets and children.

You paid for a car you do not legally own. Call the AG's consumer protection division about this dealer.

Moving to lawsuits and disputes. You are late in seeking help. You should have reported his threats and blocking the driveway to the police when...

Moving to libel/slander, so you get better answers.

Stripped of the superfluous details, you seem to have a sufficiently written agreement to enforce. Take her to small claims court and do so.