WordPress’ visual editor messing up my (nested) lists (and other things as well)

Instead of using the “type” attribute, try adding a CSS class and styling that.

<ol>
    <li>Number one</li>
    <li>Number two
        <ol class="lower-alpha">
            <li>Lowercase a</li>
            <li>Lowercase b
                <ul>
                    <li>Paragraph 1</li>
                    <li>Paragraph 2</li>
                </ul>
            </li>
        </ol>
    </li>
</ol>

Then in your CSS – depending on your site setup, you may want to add Custom CSS through the Customizer, or edit a child theme:

ol.lower-alpha {
    list-style-type:lower-alpha;
}

(Depending on your theme, you might or might not have to get more specific, but this would work for most cases.)