You have a fair amount of collection activity going on. Attempts to collect a debt are governed by a Federal law known as the FDCPA. Violations of that law (among others) by a Collection Agency (or debt buyer) can result in the Agency having to pay you damages.
Collection laws are complicated and they get confusing in a hurry. You should consider talking to a consumer protection attorney. I suggest going to http://www.naca.net/find-attorney and find an attorney in your area who handles FDCPA, FCRA, or cases involving Debt Buyers. Considering the fact that most consumer protection statutes pay for your attorney's fees, there is no reason to have to muddle through this on your own. Depending on the circumstances, many will take your case at no charge.
Junk collectors stand in the shoes of the original creditors. They have the same rights as the creditor and you have the same defenses. You are apparently trying to wear them out. It may work or it may not. These are professional. However, since most of the junk collectors have bought the loans for pennies on the dollar they may be willing to settle for a small sum.