I appreciate you returning to the thread and for performing steps to resolve the problem with the IIF file failing to load and resulting in an error. I can also see how vital it is to complete the process without issue. Let me route you to the right person to assist you further.

Since the issue persists after performing several troubleshooting steps, I recommend contacting our QuickBooks Payroll Support Team. They have the tools to pull up your account to investigate the cause of error code 3100 and the reason for the inability to load the file using an IIF.

Here's how:

1. Go to the Help menu and select QuickBooks Desktop Help/Contact Us.
2. Choose Contact Us.
3. Give a brief description of your issue about managing files using IIF and select Continue.
4. Sign in to your Intuit account and select Continue. Then, Continue with my account. If you don't already have an account, ensure to Create a new account.
5. We'll email you a single-use code. Enter your code and Continue. If you have more than one account, select the account you want to use and click Continue.
6. Choose to chat with us or Have us call you.

On the other hand, inactivating employees in QuickBooks is the option on how to de-list them from the list.

Lastly, refer to this article if you need help in changing the status of a terminated employee in QBDT: Terminate or change your employee's status on payroll.

I've got your back if you have additional questions about managing your files or employees in QBDT. I'd be glad to help you some more. Take care always, RalphScott.