How many API calls are expected to be made at peak hourly rate? This question will help determine if they have enough API limits and bandwidth for importing large volumes of data via REST API into Marketing Cloud. The REST API has limits on how many requests can be made per hour per account based on license type and edition.
Why do they require 10 years' historical data in Marketing Cloud? This question will help determine if they have a valid business case and use case for importing large volumes of historical data into Marketing Cloud. Depending on their marketing objectives and strategies, they may not need 10 years' historical data in Marketing Cloud, which could reduce their storage needs and costs.
What time of day are the API calls made is not relevant to determining a solution for importing large volumes of data via REST API into Marketing Cloud, as it does not affect how data is imported or processed. Whether someone needs to be notified if an error happens on import is not relevant to determining a solution for importing large volumes of data via REST API into Marketing Cloud, as it does not affect how data is imported or processed. Reference: https://help.salesforce.com/s/articleView?id=sf.mc_overview_large_data_extensions.htm&type=5 https://developer.salesforce.com/docs/atlas.en-us.noversion.mc-apis.meta/mc-apis/using_the_rest_api.htm

Before disconnecting the sandbox instance and connecting the production instance in the Cloud Kicks business unit, NTO should remove any existing user mappings, attribute mappings, and tracking subscriptions that are associated with the sandbox instance. User mappings define which Marketing Cloud users can access which Sales Cloud users' data. Attribute mappings define which Sales Cloud fields are used as subscriber attributes in Marketing Cloud. Tracking subscriptions define which Sales Cloud objects receive tracking data from Marketing Cloud sends. These settings should be removed before changing the CRM connection to avoid errors or conflicts. Additionally, NTO should update any query activities that reference the synchronized data extensions from the sandbox instance, as these will be different from the production instance synchronized data extensions. Reference: https://help.salesforce.com/articleView?id=sf.mc_co_disconnect_a_salesforce_org_from_a_business_unit.htm&type=5 https://help.salesforce.com/articleView?id=sf.mc_co_map_marketing_cloud_users_to_salesforce_users.htm&type=5 https://help.salesforce.com/articleView?id=sf.mc_co_create_attribute_mappings_for_your_account.htm&type=5 https://help.salesforce.com/articleView?id=sf.mc_co_set_up_tracking_information_to_send_to_salesforce.htm&type=5

Performing large-scale segmentation in Automation Studio before admitting contacts into Journey Builder can help reduce the amount of contacts admitted into the journey, thereby increasing the performance of the journey.
Creating prefiltered, sendable copies of data extensions for each journey instead of using the same entry source can also help optimize journey performance as it reduces the number of contacts that have to be processed for each journey. This can help with performance as the data extension is already filtered and therefore the journey has to do less work in terms of filtering the participants. For more information, see the Salesforce Certified Marketing Cloud Consultant Exam Study Guide[1] or the Salesforce documentation on Journey Performance[2].
[1] https://trailhead.salesforce.com/content/learn/certifications/marketing-cloud-consultant/marketing-cloud-consultant-exam-study-guide [2] https://help.salesforce.com/articleView?id=mc_jb_improve_journey_performance.htm&type=5

To use Case data from Service Cloud as part of their current email messaging campaign, Northern Trail Outfitters should use Synchronized Data Extensions on the Case Object with a Boolean filter on the Contact Me field. Synchronized Data Extensions are data extensions that allow marketers to sync data from Sales Cloud or Service Cloud objects into Marketing Cloud using filters and schedules. Using Synchronized Data Extensions on the Case Object with a Boolean filter on the Contact Me field will allow Northern Trail Outfitters to import cases that indicate the customer wants someone to contact them after the case is closed. Reference: https://help.salesforce.com/s/articleView?id=sf.mc_co_synchronized_data_sources.htm&type=5

To evaluate if a customer has made a purchase after entering the journey, a Data Relationship or Data Designer is needed to relate the master data extension and the purchase data extension. Then, a Decision Split activity can use Entry Data or Contact Data to check the purchase status. Automation Studio activities are not needed for this process. Reference: https://help.salesforce.com/articleView?id=sf.mc_co_data_relationships.htm&type=5 https://help.salesforce.com/articleView?id=sf.mc_co_data_designer.htm&type=5 https://help.salesforce.com/articleView?id=sf.mc_jb_decision_split.htm&type=5