Hello,
1- The Errors that you are mentioning regarding the Elementor, those are seem to be Warnings which might not affect your Site but if you want to probe them further then we would recommend you to contact the Elementor Plugin Support in this regard, they will guide you through as they are specifically related to Plugin.
2- Secondly, sorry for the inconvenience but actually, the new Widget style of the WP shows Warnings even with the Default WP Theme too and doesn’t work well.
So, we would recommend you to install the “Classic Widgets” plugin on your Site and then you can easily use the Widgets area as per your choice.
Here is the Plugin URL: https://wordpress.org/plugins/classic-widgets/
Best Regards