I have been with IB for over 19 years now and have never had a case where they asked me for supporting documents. Almost all my transactions cash flow wise were between either Shinsei Bank and Bank of Tokyo Mitsubishi (while in Japan) or between IB HK and HSBC. Never even once have they asked for any proof. It surprises me to hear stories like this from multiple people. Did you fund the account from obscure offshore banks or small banks perhaps?
It's a normal request nowadays.
I also have items outstanding, been trying to contact them every month to clarify some details but the CS is nonexistent, when you get someone they don't know and forward to a department that never replies.