Possible to add MIME type to web.config without possibly breaking the site?

According to this, you should remove the global setting in the special config before adding it in a different form.

Explcitly:

<system.webServer>
    <staticContent>
        <remove fileExtension=".7z" />
        <mimeMap fileExtension=".7z" mimeType="application/x-7z-compressed" />
    </staticContent>
</system.webServer>

Of course this doesn’t really help you now as you might just as well drop the local setting completely (as it’s likely to coincide with the global setting). But if you had known this back when you added local 7zip support, you wouldn’t have encountered the error now …