Hi Richard,
Thank you for your contribution to the Surface Community! I appreciate you taking the time to reach out to us. I am sorry to hear that option toggles are disabled for the primary account on Outlook. I will try my best to help you resolve the issue.
Just so that I am troubleshooting correctly, are you using the app version of Outlook or the desktop version? I am asking this because when you swipe from the right to left to open Charms in while in the app version of Outlook>tap Settings>tap Options, the
option you will see is Message list ("Group message by conversation", "Show messages from sent items in conversations", and "When an unread message is selected, mark it as read") and you shouldn't be able to change your Email signature.
Please let me know what you are seeing so that I can assist you further.