Explanation/Reference:

After the file import, using the filter activity to populate shared data extensions that are available to child BUs is the best option to ensure data is available at the child BU level. This way, the parent level BU can import the data once and then filter it based on the criteria for each child BU. The child BUs can then access the shared data extensions without having access to the parent level BU. Using the transfer file activity to move data into data extensions in child BUs would require creating separate files for each child BU and transferring them individually. Creating automations that import the data directly into the child BUs would bypass the parent level BU, but it would also duplicate the import process for each child BU.

Explanation
Einstein Copy Insights is the tool that should supply insights into the tone of subject lines. Einstein Copy Insights is a feature that uses natural language processing and machine learning to analyze your subject lines and provide suggestions for improvement. It also shows you the tone of your subject lines, such as positive, negative, neutral, or mixed, and how it affects your engagement metrics, such as open rate, click rate, etc.
Einstein Recommendations is a feature that uses artificial intelligence to provide personalized product or content recommendations for your subscribers based on their behavior and preferences. Einstein Messaging Insights is a feature that uses predictive intelligence to identify issues or opportunities with your email campaigns based on performance trends and anomalies.