Which Product enables a Marketing Cloud admin to observe customer behavior, build performance profiles and deliver the next best content?

A customer will provide a single daily file on the Marketing Cloud SFTP at 3 AM and needs an alert if the file is not present on time. The file needs to be: Imported into a staging data extension. Separated into two different data extensions.
Which workflow should meet these requirements?

Northern Trail Outfitters (NTO) is building a journey which randomly sends five different versions of an initial welcome email to new subscriber however, subscribers receive the same follow-up email two weeks later. To improve maintainability of their email content, NTO want to use 3 completely different emails, rather than having one email with dynamic content.
Which activity would allow NTO to build the journey with the fewest activities possible?

A Marketing Cloud admin wants to ensure email sends exclude their testing prefix of [PREVIEW] in the subject line when deploying from Email Studio.
What should the admin do to prevent the prefix from deploying in live sends?

Northern Trail Outfitters wants to optimize their eCommerce site by sending a follow-up Email to a customer alter an online purchase is made.
What feature could be used to solicit website feedback without navigating away from the email?