I want to edit the back end of this function. We have added custom fields in Salesforce that do not show up in the task creation flow step in Marketo. Is there a way to see the Task Validation rules?
You can't. A long while ago our SFDC admin changed the task type default from email to call, as the most most used by SFDC users. But calls have some mandatory fields (not available in Marketo of course). So tasks started to fail. So we quickly made emails the default task type again. Please vote for the idea in the comment above.