Checking on the forum the dev team have previously advised on editing the files you’ve mentioned above. See below, there’s no mention of editing the other files though. Have you tried without the script you’ve added and the changes to the controlpanel.php? Just with the font-replacement.php code?
The best way to add another font would be to add it to the front-replacement.php file and overwrite the other fonts. You could indeed use a plugin for the sliders, but you’ll need to either insert it on the page itself, or code it into the header. I’d only advice this if you have experience with php/html/css.