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You can try to use a cache plugin, like wp super cache.

To install a theme is very easy, just upload it to the themes directory, then activate it at the backend. But it will be difficult if you want to code it yourself.

There are lot's of theme may meet your needs, but if you want it looks exactly like the one, you need some more coding for that. Here is a collection of 60+ themes, hope you can find the one you like: ColorVila WordPress Themes

Have you fixed this? I thinks you didn't get the right image size. Please have a check this article https://codex.wordpress.org/Function_Reference/add_image_size

I don't think you need another DB, Wordpress has the ability to handle heavy load, you can find some plugins for this.

The pmpro_https_filter() function hasn't been defined. You need to check the code of the plugin, or ask the plugin author for help.

I think you can post the error to woocommerce forum, where you can get more help

There is a bug for saving the attributes of WC. I have met this before, it need to modify the code of WC, but it isn't recommend, once you upgrade WC, the code will be lost.

I like static html, but WordPress is more flexible.

It depends on what ecommerce plugin you're using. Please post your link here.

I'm not a fan like to install lots of plugins, which will make your site slow. So I never used jetpack before. Bug for a beginner, it is very helpful.

It seems there is no functions to save the options. Maybe you haven't copied all the codes. And this function "update_option( 'sa_options', $sa_options)" is the one you lost, which has the function of saving options. You should find the code segment include this.

You can contact the author of the themes for help. Usually the premium themes will supply several color schemes. And if you know css, it will not be a problem to change the color.

WordPress is best for personal user and small site. Especially the huge resource, you can find all kinds of free and premium themes, plugins.