[Theme: Hueman] Image scaling/thumbnails

WordPress 4.4.2
Hueman Child theme
Style.css
Site

The problem
Basically the idea is that I want to upload a full size image (1200×800), have either hueman or WP create a 600×400 ‘medium’ copy, then have that copy scaled to 300×200 for the gallery thumbnails. I can then link those thumbnails to the full size image in a lightbox. I managed to get this to happen exactly how I wanted, but only on desktop/large screens. The gallery looks terrible on mobile screens, all compressed and tiny.

What I want to achieve
1. To have complete control over image scaling, and the number and size of copies made when I upload an image. (Also to have a better understanding of this process!)
2. To be able to upload a ‘full size’ image at 1200×800, and have a 600×400 copy made by WP or the theme which I can then use as gallery thumbnails scaled to 300×200 (to look nicer).
3. For the gallery to look nice on mobile which it currently does not.

Here is a page that looks good on desktop but bad on mobile.

What I’ve done so far
1. Added some CSS to child styles:

.entry .gallery img {
    height: 200px;
    width: 300px;
}

2. Added to child functions.php (to disable thumbnail upscaling)
function alx_thumbnail_upscale() {}
3. Unselected the options in Media > Settings so that only a Thumbnail (150×150) is generated (so I can see a preview of media in the media library), and medium is generated (600×400).

This originally got around an issue I was having where thumbnails would look really bad because they were being scaled. The combination of the medium image and CSS I added achieve what I wanted perfectly, but only when viewed on a large screen.

My questions
1. How do I achieve what I want while still having it behave nicely on smaller screens?
2. Can I untick all the options in Settings > Media and have Hueman handle all of the image generation through the ‘Thumbnail Sizes’ option in the alx_setup() function?
3. How do I edit that function to achieve what I want?

I have read the theme documentation, read similar threads, read some codex pages, but I don’t understand how to get from where I am now to where I want to be.

I greatly appreciate any help! Thank you.