UPDATE:

I called their US toll free number yesterday through Skype at 8pm and took around 15 minutes to get through. The customer service rep sounded confused at first when I say I need to perform verbal verification because he said my account is already individual account type, meaning the delinking process is complete. He also said if we want to check whether delinking is complete, then just see if your account type is "Individual", because that is how they check as well. So at the end of the day, I didn't have to do any verbal verification even though I got the email...which is weird.

He sounded like he hate his job and I can understand that, being a customer service rep is not easy

Full process for me:
1. Sent email with form and passport to globalsupport@tradestation-international.com with subject "U****** De-linking Request" on Wednesday, 21 July, 9:15pm. I also sent the same email to accountmanagement@interactivebrokers.com at the same time.
2. Got an email from IBKR on Thursday, 22 July, 7:33pm to perform verbal verification
3. Forgot to do the verbal verification but logged in my IBKR account on Saturday, 24 July and realized got "Pro" logo, "Refer a friend", and account type is "Individual". Also can change to tiered pricing.
4. Called IBKR on Monday 8pm and customer service rep told me don't need do verbal verification because delinking process is complete.