There are three ways the Marketing Manager can create a new prospect in Marketing Cloud Account Engagement. They are:
Click the "Send to Account Engagement" button on a Salesforce Lead. This option allows the Marketing Manager to send a one-to-one email to a lead in Salesforce using an Account Engagement email template. If the lead does not already exist as a prospect in Account Engagement, it will be created automatically and synced with Salesforce1 Manually add a prospect in Marketing Cloud Account Engagement. This option allows the Marketing Manager to create a new prospect record in Account Engagement by entering the prospect's email address and other information. The prospect can then be added to lists, campaigns, or engagement programs2 Import a .CSV file with the prospect's email address. This option allows the Marketing Manager to import a list of prospects from a .CSV file into Account Engagement. The file must contain the prospect's email address as the first column, and can also include other fields. The imported prospects can then be assigned, tagged, or added to lists or campaigns

According to the Salesforce documentation, the two prospect activities that trigger a sync from Marketing Cloud Account Engagement to Salesforce are: A) Submitting a form, and B) Opening an email. A form submission is an activity that occurs when a prospect fills out and submits a Marketing Cloud Account Engagement form or a form handler. An email open is an activity that occurs when a prospect opens an email sent from Marketing Cloud Account Engagement. These two activities will trigger a sync from Marketing Cloud Account Engagement to Salesforce, meaning that the prospect's information and activity data will be updated in Salesforce. A sync from Marketing Cloud Account Engagement to Salesforce happens when a prospect is created or updated in Marketing Cloud Account Engagement, or when a prospect performs a significant activity in Marketing Cloud Account Engagement. Clicking a custom redirect or unsubscribing from email are not activities that trigger a sync from Marketing Cloud Account Engagement to Salesforce, as they are not considered significant activities in Marketing Cloud Account Engagement. A custom redirect is a trackable URL that can be used to measure and report on the clicks of any online content. An unsubscribe is an action that occurs when a prospect opts out of receiving emails from the sender. These two activities will not trigger a sync from Marketing Cloud Account Engagement to Salesforce, unless they are combined with other activities that do trigger a sync, such as submitting a form or opening an email. References: Salesforce documentation

If a marketing manager wants to view list email engagement metrics in Engagement History in Salesforce, they will not be able to see the Total Replies metric (A). This metric is not supported by Engagement History, which only shows the following metrics for list emails: Total Delivered, Unique Opens, Unique Clicks, Hard Bounces, Soft Bounces, Opt Outs, and Spam Complaints. The other metrics (B, C, D) are supported by Engagement History and can be viewed in Salesforce. Reference: Salesforce Engagement History: A Complete Overview

The automatic workflow that can be created to prevent the unassigned prospects from getting assigned is to create an automation rule based on the prospects' time to adjust their score to 0 if they haven't been active in
60 days. An automation rule is a rule that runs continuously in the background and matches prospects based on the criteria you set. You can use an automation rule to perform actions on the matched prospects, such as adding them to a list, assigning them to a user, or changing their field values. However, an automation rule can only match a prospect once in its lifetime, even if the prospect meets the criteria again later. This prevents duplicate or conflicting actions from being applied to the same prospect1. Therefore, an automation rule is ideal for creating a workflow that resets the score of the unassigned prospects who have been inactive for 60 days, and prevents them from getting assigned by another automation rule that assigns prospects based on their score. To create an automation rule, you need to specify the criteria and the actions that you want to execute. For example, you can create an automation rule that matches prospects who have not been assigned to a user, have not had any activity in the last 60 days, and have a score greater than 0, and then use the action Adjust Score to set their score to 0.

The metrics in the list email report that could help the Account Engagement user understand how these prospects may have become unmailable are Total Opt Outs and Hard Bounces (D). These metrics show the number and percentage of prospects who either opted out of receiving future emails or had their emails bounced back due to a permanent error, such as an invalid email address. These prospects are marked as Do Not Email and cannot be emailed again unless they opt back in. The other metrics (A, B, C) are not directly related to the prospects' mailable status, but rather to the email delivery, performance, and tracking. Reference: List Email Report Metrics