If you cannot delete an extension then you likely aborted the create wizard and you don’t have any versions on the extension. The extension needs at least one version before you can delete it.
Each Client ID is specific to the extension or application.
So if you delete it and make a new extension/application you’ll get a new ClientID.
Extension Client ID"s are also used in the URL to display and install the extension so this information is public. If you are building an extension there is no need to delete, just got create a version and get building.