Having a little difficulty in understanding your question, but I'll try and come up with suggestions.
Keeping things simple, a Command object's main purpose is to represent user input in an object oriented form. It makes it easier to work with data using objects (similar to mapping data to Entity objects) within the Java program (such as updating the database, validation, etc.).
In the given scenario, if you were to hold a list of replies for a post, then that would be held as a Value object. So this would be separate to the Command object. Override the referenceData(HttpServletRequest request) method to assign Value objects to the page when it loads.
I hope I've understood some of your questions. Now for my question. Why is a Command object called a "Command" object? Is it derived from the Command pattern that is used by these Controllers? I just wanted to know why it is named like this.
(Btw, I cannot provide an answer to question 2).