Adding a pop-up, editing text spacing, etc

Hey all– several wide-ranging/unrelated questions re: the Cubic theme on WordPress.

Context: our site (livingwithoilandgas.com) is hosted on WordPress, so we can’t add plugins. And we do have the Premium plan, so we have the ability to add to/edit the CSS.

Questions:

1. Is there a way to create a pop-up, for when visitors first land on the homepage, without being able to install a plug-in?

2. The text in my posts is double-spaced in some places, and not spaced out enough in others (see: https://livingwithoilandgas.com/2016/01/27/belfry-mt/ vs. https://livingwithoilandgas.com/about/). Does anyone know how I can adjust the first to not be double spaced, and the second, to have space between paragraphs?

3. Is there a way to either expand the width of a post (see: https://livingwithoilandgas.com/about/ and how it’s more or less contained to a narrow column in the middle of the website), or to more flexibly arrange photos & text? As in, could I put the text next to the photo, rather than just above or below?

4. Is there a way to insert a gallery/slideshow of photos in a single post?

Thanks!