Customize Sharepoint Online : how to make SPfont work properly?

Question

I'm facing a weird issue with my spfont file:

In the Change the look menu of my site (which is not the root btw), where I can change the background, masterpage, theme and fonts, the preview does not work anymore as soon as I select my custom font, and displays "undefined" instead. No more preview, just "undefined". And this occurs only when I select the custom spfont, not any other element.

I found out that still, if I click "Try it" anyway, the font works in the website and SharePoint generates all the font-faces needed in his corev.css. Correctly? Idk. Probably not, actually.

Because after that, I also found out that some weird stuff was happening: in Site settings, hovering one of the links will make it take less space. Actually, it will make the <li> smaller, while the <a> inside is at auto for both height and width. The consequence is that, when you come to the page you've got pretty readable links, but when you hover one of the lists from top to bottom, each element shrinks one by one, making everything stuck to each other and the list takes twice less space than it used to.

I've dug deep inside my css, only to find that the problem wasn't there: removing my custom spfont fixed it. Here is the (truncated) spfont file:

<?xml version="1.0" encoding="utf-8"?>
<s:fontScheme name="Oswald" previewSlot1="title" previewSlot2="body" xmlns:s="http://schemas.microsoft.com/sharepoint/">
    <s:fontSlots>
        <s:fontSlot name="title">
            <s:latin typeface="Oswald Regular" 
                eotsrc="_layout/15/Oswald/OswaldReguar.eot" 
                woffsrc="_layout/15/Oswald/OswaldReguar.woff" 
                ttfsrc="_layout/15/Oswald/OswaldRegular.ttf" 
                svgsrc="_layout/15/Oswald/OswaldReguar.svg"
                largeimgsrc=""
                smallimgsrc="" />
            <s:ea typeface="" />
            <s:cs typeface="Oswald Regular" />
            <s:font script="Arab" typeface="Segoe UI Light" />
            [& many other lines for many countries, identical to the spfont files given by SharePoint]
        </s:fontSlot>
        <s:fontSlot name="navigation">
            <s:latin typeface="Oswald Light" 
                eotsrc="_layout/15/Oswald/OswaldLight.eot" 
                woffsrc="_layout/15/Oswald/OswaldLight.woff" 
                ttfsrc="_layout/15/Oswald/OswaldLight.ttf" 
                svgsrc="_layout/15/Oswald/OswaldLight.svg"
                largeimgsrc=""
                smallimgsrc="" />
            <s:ea typeface="" />
            <s:cs typeface="Oswald Light" />
            [Same as above]        
        </s:fontSlot>
    <s:fontSlot name="large-heading">
    <s:latin typeface="Oswald Regular" 
        eotsrc="_layout/15/Oswald/OswaldReguar.eot" 
        woffsrc="_layout/15/Oswald/OswaldReguar.woff" 
        ttfsrc="_layout/15/Oswald/OswaldRegular.ttf" 
        svgsrc="_layout/15/Oswald/OswaldReguar.svg"
        largeimgsrc=""
                smallimgsrc="" />
            <s:ea typeface="" />
            <s:cs typeface="Oswald Regular" />
            [Same as above]
        </s:fontSlot>
    <s:fontSlot name="heading">
            <s:latin typeface="Oswald Light" 
                eotsrc="_layout/15/Oswald/OswaldLight.eot" 
                woffsrc="_layout/15/Oswald/OswaldLight.woff" 
                ttfsrc="_layout/15/Oswald/OswaldLight.ttf" 
                svgsrc="_layout/15/Oswald/OswaldLight.svg"
                largeimgsrc=""
                smallimgsrc="" />
            <s:ea typeface="" />
            <s:cs typeface="Oswald Light" />
            [Same as above]
        </s:fontSlot>
        <s:fontSlot name="body">
            <s:latin typeface="Arial" />
            <s:ea typeface="" />
            <s:cs typeface="Arial" />
            [Same as above]
        </s:fontSlot>
    </s:fontSlots>
</s:fontScheme>

So there's definitely something strange with this file, but can't find what. I repeat that it "works", kind of. At least the font files are found, as well as the smallimgsrc and largeimgsrc before I removed their URL to see if that would change anything. It didn't. So it works, but in a shitty way. And I would like to make it work... well, normally if possible, since the goal is to create a conception package to export on other websites. Is there something messy in here that I don't see? Why does it behave like this?

Do you think it could have something to do with this update of February, maybe?

Any help appreciated =/