Forecasting is a feature that allows you to predict and plan the sales cycle from pipeline to closed sales, and manage sales expectations throughout your organization. Opportunity stages are the steps that an opportunity goes through as it moves from creation to close, and they determine the probability and forecast category of the opportunity. References:
https://help.salesforce.com/s/articleView?id=sf.forecasting3_overview.htm&type=5
https://help.salesforce.com/s/articleView?id=sf.customize_opptystages.htm&type=5

To identify new leads entered by interns from existing leads, the administrator should define a record type and assign it to the interns. This will allow them to select a different record type when creating leads, and distinguish them from other leads based on record type. Creating a separate Lead Lightning App or updating the active Lead Assignment Rules will not affect lead identification. Setting up Web-to-Lead form will not work if the interns are entering leads manually in Salesforce. References:
https://help.salesforce.com/s/articleView?id=sf.customize_recordtype.htm&type=5

Dependent picklist and global picklist are two options that can be used to meet this requirement. Dependent picklist allows users to create a conditional relationship between two picklist fields, where the available values in one field depend on the value selected in another field. Global picklist allows users to create a set of picklist values that can be shared across multiple fields and objects, ensuring consistent values and reducing maintenance. References:
https://help.salesforce.com/s/articleView?id=sf.fields_about_dependent_picklists.htm&type=5
https://help.salesforce.com/s/articleView?id=sf.picklist_global_picklists.htm&type=5

Email-to-Case allows administrators to set up routing addresses that automatically create cases from incoming emails and assign them to queues based on predefined criteria. This way, emails from customers can generate cases automatically and be sent to the correct queue. A flow is a tool for building automated processes, but it is not designed for email routing. A custom email service is a way to process inbound emails using Apex code, but it requires coding skills and is more complex than Email-to-Case. An escalation rule is a way to escalate cases based on certain conditions, but it does not create cases from emails or assign them to queues.
References: https://help.salesforce.com/s/articleView?id=sf.customize_email2case.htm&type=5

Case escalation rules allow you to escalate cases based on certain criteria, such as time or priority.
References: https://help.salesforce.com/s/articleView?id=sf.customize_escalation.htm&type=5