I would go for these themes:
https://themeforest.net/item/jevelin-multipurpose-premium-responsive-wordpress-theme/14728833
https://themeforest.net/item/soledad-multiconcept-blogmagazine-wp-theme/12945398
https://www.elegantthemes.com/gallery/divi/
You can use the following plugin to completely hide your WordPress footprints:
https://codecanyon.net/item/hide-my-wp-amazing-security-plugin-for-wordpress/4177158
Unfortunately, I am not familiar with such plugin. But it sounds like a custom made plugin.
Maybe this plugin is useful to you?
https://wordpress.org/plugins/epoll-wp-voting/
I am happy to answer your questions

:
1. That's hard to determine without knowing the theme name or plugin. If you downloaded the plugin or theme from a shady website, it might be a mal-ware backdoor.
2. The Divi Builder comes with a few lay-outs which you can use? Also, I have worked with
this website before and it's getting more easier to create a neat landing page.
3. The new WP backend? Do you mean the new WP Editor? If so, you're able to install the 'Classic Editor plugin' to revert back to the 4.9 editor.
Sorry, I am not aware of such plugin. If it comes to guest posting, no script or service will be able to verify each post before it goes live according to your requirements. Your best call would be by hiring a PA to check all the submitted guest posts. What I do with completely new themes: I install the theme on two environment: one live and one local. On the local theme, I am importing all the demo content and adjust it to my needs and push it to the live environment.
https://mega.nz/#F!Iep2XS6A!RurMqmUHtAvDc5cw6h5nVg
Are we talking about the same products or you want to manage WooCommerce products from multiple websites from one central place?
Yup, I know the basics of coding and I have been involved in many developments of themes and plugins. So A/B testing, testing and/or staging environments are familiar to me.
What exactly would you like to test on a testing environment?
Neat websites and your best call would be a theme like this:
https://themeforest.net/item/wp-rentals-booking-accommodation-wordpress-theme/12921802
Have you checked
WP Robot already?