The Content Split activity should be used for multivariate content testing within Journey Builder. This activity allows the journey to send different versions of email content to different audience segments and measure their performance based on open or click rates. The other activities are not suitable for multivariate content testing.

When incorporating custom brand fonts into emails for Northern Trail Outfitters:
Web-safe Fonts: It's recommended to use web-safe fonts in emails to ensure consistent rendering across different email clients. Custom fonts are often not supported by many email clients, which can lead to inconsistent branding and user experience.
Font Selection: Choose a web-safe font that closely resembles the custom brand font to maintain the brand's visual identity while ensuring the email content is accessible and readable across all platforms.
Reference: Salesforce Marketing Cloud Email Design Best Practices

For a non-technical marketing user to manage a campaign sending renewal reminders:
Data Filter: Use Data Filters to segment customers based on the expiration date included in the data file. This enables targeting customers 15 days and 7 days before subscription expiration.
Template-based Emails: Implement template-based emails to allow the marketing user to easily update and manage email content without needing technical skills or administrative rights.
Automation Studio: Utilize Automation Studio to schedule and automate the sending of these emails based on the specified criteria, streamlining campaign management.
Reference: Salesforce Marketing Cloud Automation Studio

For a Membership Renewal drip campaign with a master audience updated in real-time:
Filter Activities: Use Filter Activities to segment the master data extension based on the renewal date criteria-one month prior, one week prior, and on the renewal date.
Send Activities: Follow each Filter Activity with a Send Activity targeting the specifically filtered audiences, ensuring that each email is sent at the correct time relative to the member's renewal date.
Reference: Salesforce Marketing Cloud Automation Studio

Einstein Email Recommendations is a product that enables a Marketing Cloud admin to observe customer behavior, build performance profiles and deliver the next best content. This product uses predictive intelligence to recommend products or content based on each customer's preferences and actions. The other products do not have this capability.