Car was repossesed. They are asking for full amount due not just past due. is that legal?

Most contracts which involve installment payments contain what are known as "acceleration clauses", which provide that if there is a default the entire amount becomes immediately due and payable.  If you look at your contract, I bet you will find it there.  As for a breach of the peace, that is a matter for the police, but you probably won't get far complaing about a past breach of the peace, rather than an ongoing one.