To meet the requirements, the Organization-Wide Default (OWD) for accounts should be set to Private to restrict access to only the owners of the records by default. Criteria-based sharing rules can then be created to grant access to the specific accounts for the respective North American and European marketing teams, ensuring they only see accounts within their regions. For the Inside Sales Team, a profile with the 'View All' setting for accounts ensures that they can see all accounts in Salesforce. This approach ensures that access is appropriately restricted and shared based on roles and regions.

Users can access a Visual Workflow through a Custom Button and a Quick Action. Custom Buttons provide a straightforward way for users to execute a flow directly from a record page or list view. Quick Actions allow users to start a flow from the publisher on the record page or from the global actions menu. While a flow can indeed be triggered by a Process, this is done automatically and not manually by the user, hence it doesn't fit the criteria of being accessed by users directly.

When creating a Student Lookup field on the Training object, it's important to note that Lookup fields can be made optional, meaning a Training record can exist without being linked to a Student. Additionally, cross-object field updates are not supported between Training and Student records in a Lookup relationship; such functionality is available in a master-detail relationship but not for lookups by default.

To accomplish roll-up summaries in Salesforce, appropriate field types should be selected based on what can be summarized. Roll-up summaries can perform operations such as SUM, MIN, MAX, and COUNT. Percent and Currency fields can be summed to provide meaningful totals, and Date fields can be used to find the earliest or latest values, making Percent, Currency, and Date suitable for these summarization purposes.