Hello,
01. You are referring to two different layouts:
Theme Default Layout – This layout follows the settings available in Theme Settings > Blog > Single Post. You can configure the default design from there. Here is the path to access these settings. https://ibb.co/W4nfmBV4
Custom Layout for Single Post – You can create and use a custom single post layout with Elementor Pro. In this case, the header and footer will remain the same as the theme’s default. However, you might need to select the sidebar manually from the post Settings when creating your custom layout.
02. Try using the Sidebar-related widget in your Elementor layout to check if the issue is resolved. If the problem persists, please share your WordPress login details so we can investigate further.
Best Regards.