Dart-SCSS, Autoprefixer

There are a lot of questions that needs answering (and I'm still looking into) since this is still a rough build imo. Below are some of them:

1. What would be the best fallback for gap so the site will still work in IE?
2. What is the best way in handling svg so the site will perform better? Currently, the svg's were handled using <img> tags which for sure is not the best way of doing it. Should I use it as a background-image instead or just use the svg codes?
3. How to better use fontawesome? Currently, a link to fontawesome is used. This method is too dependent on the site. Including the entire library is counter-productive because the lib includes files that will not be used in the project. Would using the svg codes be better for performance and is it even allowed by fontawesome?
4. I encountered an issue with :last-child, :nth-child() and :nth-last-child() on a floated element so I opted to use :last-of-type instead. Was I not understanding these pseudo classes or is this a known issue?