Hi William,
Thanks for the very insightful post. I want to reiterate the disclaimer of modifying the customizations.xml file as mentioned in the blog article.
With that disclaimer, I would recommend using a tool like the Stunnware Tools to help you build FetchXML. You can find that tool here:
You're going to need to add the link entity for Contact to Account. Essentially you want to retrieve Activities for an Account and the related Contacts. It would look something like:
<entity name="activitypointer">
<link-entity name="account" from="accountid" to="regardingobjectid">
<link-entity name="contact" from="parentcustomerid" to="accountid" />
</link-entity>
</entity>
However, I would use the tool which will allow you to return real results while your building your FetchXML.
Hope this helps!