Thanks for taking the time to answer my questions, I'm really appreciative for your help!
I've setup IIS so that it is it's own site now, and loads by visiting the root of the server (no need to go to /Kentico anymore).
Using a relative path loads fine for all sites, so long as I am logged into through /Admin in that browser. Once I log out however I get the following errors:
In Chrome: E4T:1 Failed to decode downloaded font
In Opera: Failed to decode downloaded font:
In Firefox: downloadable font: rejected by sanitizer
In IE: CSS3111: @font-face encountered unknown error.
Using the {%RootDocument.AbsoluteURL|(identity)GlobalAdministrator%}
Macro causes different errors:
In Opera and Chrome(same while logged in): Access to Font at 'http://localhost/App_Themes/Default/Custom/slick.woff' from origin 'http://<i>' has been blocked by CORS policy: No 'Access-Control-Allow-Origin' header is present on the requested resource. Origin 'http://<ip>' is therefore not allowed access. The response had HTTP status code 404.
In Firefox (same while logged in): Cross-Origin Request Blocked: The Same Origin Policy disallows reading the remote resource at http://localhost/App_Themes/Default/Custom/slick.ttf. (Reason: CORS header ‘Access-Control-Allow-Origin’ missing). (unknown)
downloadable font: download failed (font-family: "slick" style:normal weight:normal stretch:normal src index:2): bad URI or cross-site access not allowed source: http://localhost/App_Themes/Default/Custom/slick.ttf
No IE error, but the font doesn't load either.
You mentioned to leave the font files where they should be, I'm not sure where that would be, maybe that could be a factor? The files were not where I'd expect them to be based on the official docs recommendations (I should note that I'm working on an inherited project), instead it was in a folder that mirrored a css stylesheet's name in the App_Themes dir. Even then, moving the files around and changing the path loads the font as expected, as long as I am logged in as an administrator on the browser that I am viewing the page with the font on.
Does Kentico have a specific way through it's CMS UI to load custom fonts? Would moving font files and creating directories in the file explorer break anything in Kentico?