The following activities would make a prospect active in Marketing Cloud Account Engagement:
Submitting a Marketing Cloud Account Engagement form, which indicates the prospect's interest and consent to receive communications from the company.
Clicking on a custom redirect, which tracks the prospect's engagement with external links or downloadable content.
Visiting a Marketing Cloud Account Engagement-tracked page, which captures the prospect's browsing behavior and page views on the company's website. Reference: [Marketing Cloud Account Engagement Prospect Activities]

According to the Salesforce documentation, the two form field options that should be enabled to make the "Comments" field visible and empty every time a prospect returns to one of the forms are: B) Always display even if previously completed, and C) Maintain the initial value upon subsequent form submissions. The Always display even if previously completed option ensures that the field is not hidden or pre-filled with the previous value when the prospect revisits the form. The Maintain the initial value upon subsequent form submissions option ensures that the field is not overwritten with the new value when the prospect submits the form again. These two options combined will make the field appear as visible and empty every time the prospect returns to the form. The Do not prefill option will prevent the field from being pre-filled with the prospect's information from Marketing Cloud Account Engagement or Salesforce, but it will not prevent the field from being pre-filled with the previous value from the same form. The Display other fields in this form based on the value of this field option will create a dependent field that will show or hide other fields based on the value of the "Comments" field, but it will not affect the visibility or emptiness of the "Comments" field itself. Reference: Salesforce documentation

According to the Engagement Studio FAQ, when a program is paused, the prospects in the program are also paused and their wait times are not affected. Therefore, when the program is restarted, the prospects resume from where they left off. In this case, the prospect was on day 3 of a 5-day wait time when the program was paused, so they will be on day 3 when the program is restarted.