Microsoft 365 features that help users manage their subscriptions, account settings, and billing information.
If I cancel Microsoft 365 Personal and continue using the Family plan with the same Microsoft account, will all my existing OneDrive files and folder structure remain intact?
Nothing will change every the license. Your OneDrive (and email) won't be affected. At most, you would need to sign in to activate office again.
You can't accept the share until the personal subscription expires and they won't give you a refund unless you just renewed it. Cancelling the personal subscription won't end it now but at the current expiration date.