AppExchange is a marketplace where app builders can find prebuilt resources to help with migration and adoption projects. AppExchange offers apps, components, solutions, and consulting services that can assist with moving from Classic to Lightning Experience. Import Wizard, Lightning Object Creator, and Flow Builder are not sources of prebuilt resources, but tools for data import, object creation, and automation respectively.

The formula that should be used to select the contact's preferred name for email communications is IF(NOT(BLANKVALUE(Nickname__c)),Nickname__c, FirstName). This formula checks if the Nickname__c field is blank or null, and if not, it returns the value of the Nickname__c field. Otherwise, it returns the value of the FirstName field.

The app builder should use Web-to-Case to enable visitors to the website to open cases. Web-to-Case is a feature that allows users to create an HTML form that can be embedded into a public-facing website. When visitors fill out and submit the form, a new case is automatically created in Salesforce with the information from the form fields. Web-to-Case can also assign cases to queues or users based on predefined rules and criteria. Outbound message is not a valid tool, as it does not enable visitors to open cases. An outbound message is a type of workflow action that sends a secure XML message to a designated endpoint URL when a record meets certain criteria. An outbound message can be used to integrate Salesforce with external systems or applications. Screen flow is not a valid tool, as it does not enable visitors to open cases from a public-facing website. A screen flow is a type of flow that allows users to create an interactive guided process that can collect, display, and update data from multiple objects within Salesforce. A screen flow can be embedded into Lightning pages or Visualforce pages, but it cannot be exposed to unauthenticated users on a public website.
Email-to-Case is not a valid tool, as it does not enable visitors to open cases from a website. Email-to-Case is a feature that allows users to create cases from email messages sent to a specific email address. Email-to-Case can also capture email attachments and thread information and associate them with the case.

Setting email deliverability to no access would prevent outbound email sends during testing in a sandbox.
This would ensure that no emails are sent to any users or external email addresses from the sandbox. Setting email deliverability to system email only would still allow some emails to be sent, such as password reset emails or new user emails. Configuring SMTP authentication or email relay would not affect email deliverability in a sandbox

The best feature to use to ensure consistent case priority entry is Path. Path allows users to see key fields and guidance for each stage of a process, such as a case lifecycle. Path can also enforce data quality by making certain fields required at each stage. See [this article] for more information on Path.