What do I now about this collection agency?

Logn story short...I went to ITT tech for 2 years getting loans for the whole thing. It took me around 3 years to get a job even remotely related to what I went to school for and it took me closer to 5 or 6 years to get a good paying job in that field. Well after the 6 month grace period was up after graduation I liked to choked when I got student loan bills in the mail for $500+ a month. Almost as much as my rent. I contacted them and they said I could consolidate all my loans and put a hardship forbearance on them until I could afford to pay for them so I did. What they failed to mention was the fact that they could only do that with my government loans and not with the private loans I had taken out. Unfortunately I didnt know this since I was poor for the first few years after graduating I was moving from one place to another to another. If they were sending me collection notices I wasnt aware of them cuz I never received them.

So I eventually landed a good paying job at an air force base as a contractor which required me getting a security clearance. As part of getting my clearance I had to go before a federal judge and explain why I had so much debt that hadnt been paid. (There were also some small things on my credit report...first time I had ever pulled my credit report since no one ever really taught me about such things growing up). I managed to convince the judge that I wasnt a security risk by starting to pay the collection agency months before my court date and got my clearance.

So I paid this collection agency (Windham Professionals) $300 a month for about 2 years even though they were saying i owed them 23K for 9K worth of original debt. I never agreed to amount but started paying them for fear of loosing my good job. Well after paying them for a couple of years....something like 6K I noticed my Windham professional statements that I recieved 2 or 3 times a year only ever showed the amount i owed and nothing else. Well after 2 years and 6K later the statement still said I owed only 1800 dollars less than the original amount. I called them to ask them about this which got me no where and got me so angry I looked up stuff online and found a generic validation letter so I sent it to them. They disappeared. This kind of surprised me as Salie Mae sent me to them so I figured they could validate the debt but I figured why not? right? Well since then 2 other collection agencies tried to collect which I just ignored and never herd from them but once or twice.

Then Convergent sent me a rather lengthy collection letter so I figured Id try the ol validation letter again on them just to make sure they could collect the debt and again asking for specific tallies of how they figured the amount that i owed them etc as they are claiming I owe them 23K and some change now too. Well they sent me a reply back to my letter but all they sent me was all the paperwork I had signed to get the original loans both private and government backed and a copy of the paperwork showing that if I defaulted on the loans then I could be responsible for paying a collection agency. Now my state does not require a license for collection agencies to collect in Ohio so I guess that part doesnt matter but there is nothing outside of all the original paperwork I signed to get the loans from Sallie mae. Is this all that is required for validation? I still have no idea how they arrived at the total amount they say is due, they never stated what the loans were originally for, They also dont show any records of the payments I made to the other collection agency or even any paperwork from the other agency I was paying for awhile, and they have not proven the statute of limitations hasnt expired. So I have 2 questions at this point.

1) Did they completely validate the debt?
2) If not then what should I do from here?