KimS, the receipt of United Legal Corp not responding to your validation request is an important proof that makes your case stronger towards paying the debt. Keep it in safe place as it might be required later. If they are listed in your credit report, you can dispute with the bureau on this item with the explanation. The bureau will conduct an investigation and verify the item with them.
Follow the same process with Arrowhead Investments also asking for debt validation. If they are the ones that are holding your account legally, they will honor your request. As said, send your letter through certified mail with return receipt requested and keep copies documented.