Not able to set the proper column permissions on Activity object

Hello,

I have a requirement for Activity. All fields should become Read Only when the status of the Activity is 'In Progress' until the Approval from Head of Sales is received. (This is working as the Change Access Rights has been set in Business Process)

After the Head of Sales Approves only 'Complete' field should be editable by a Sales Executive and not any other field.

I have changed the access in Change Access Rights after Head of Sales approves (by providing the edit access to the Sales Executive in Business process. However, this is letting the Sales Executive to edit the Account field also. 

The Business Rules applied to lock the Account is not working although the rules are working fine on the Start, Due and Subject fields (attached is the screenshot)

I have set the Column permissions as attached.

Any input on this would be greatly appreciated.

Thanks in advance.

Madhuri