Looping Related Entries with a Super Table
The issue is: you don't really loop your entries at all.

{# get all super table rows #}
{% set superTableRows = block.related.all %}

{% if superTableRows is not empty %}
    &lt;div class="{{ padding }} {{ fontsize }}" id="{{ blockID }}"&gt;
        &lt;div class="{{ margin }}"&gt;

            {# loop all of your super tables #}
            {% for row in superTableRows %}

                {# loop all contentEntries in each row #}
                {% for entry in row.contentEntry.all() %}

                    {# 
                        display the alternative title set in your supertablerow 
                        and if thats not set display the entries title
                    #}
                    {{ row.altTitle?? entry.title }}

                    {# same as above #}
                    {{ row.altDescription ?? entry.previewDescription }}

                {% endfor %}
            {% endfor %}
        &lt;/div&gt;
    &lt;/div&gt;
{% endif %}


block.related is an Object of type ElementQuery. block.related.altDescription returns the criteria of this Query, this is mostly empty since no criteria is set by default.