Explanation
To report on 'Not Sent' contacts in Journey Builder, Northern Trail Outfitters should use the standard report
'Subscribers Not Sent to'. This report shows how many contacts were not sent an email due to various reasons, such as unsubscribes, bounces, duplicates, or errors. References:
https://help.salesforce.com/s/articleView?id=sf.mc_rn_subscribers_not_sent_to_report.htm&type=5

This setting allows the consultant to verify the sender's identity and configure the incoming email settings.
Reference: https://help.salesforce.com/articleView?id=mc_es_sap_overview.htm&type=5 To redirect its emails to Marketing Cloud servers for processing, Northern Trail Outfitters should use a Sender Authentication Package. A Sender Authentication Package is a feature that allows marketers to authenticate their email sends with their own domain and branding, instead of using Marketing Cloud's default domain and branding. This will improve email deliverability and reputation, as well as provide more control over email settings and preferences.References:https://help.salesforce.com/s/articleView?id=sf.mc_es_sender_authentication_package.h

To connect relational data into their journey without overloading the entry data, Northern Trail Outfitters should use Data Designer. Data Designer is a tool that allows marketers to create attribute groups that link different data sources in Contact Builder. This way, they can access relational data in decision splits and other journey activities without having to include all the data in the entry source data extension.References:https://help.salesforce.com/s/articleView?id=sf.mc_co_data_designer.htm&type=5

Explanation
Synchronized data extensions are data extensions that are automatically populated with data from Sales Cloud objects using Marketing Cloud Connect. They can be configured with filters to limit the records that are imported based on criteria such as field values or record types. By using synchronized data extensions with a filter on the consent checkbox field, NTO can import their Sales Cloud data into Marketing Cloud and leverage it within Journey Builder. Synchronized data extensions can be refreshed as frequently as every 15 minutes, which meets the requirement of having no more than 30 minutes delay. References:
https://help.salesforce.com/articleView?id=sf.mc_co_data_integration_synchronized_data_sources.htm&type=5
https://help.salesforce.com/articleView?id=sf.mc_co_implement_synchronized_data_sources_best_practices.htm