How to retrieve all meta data directly from the $post object? Such as $post->related_topics?

By calling $post->related_topics you are using the magic functions of the object ( __get and __isset )

To make it simple, behind the scenes it is the same as :

get_post_meta( $post->ID, 'related_topics', true );

So you are just abstracting the use of the function get_post_meta. And as you can see, the third argument is defined as TRUE. And this tells the function to return a single value.

So its ok if your meta is a single value, but if you have multiple values, then you must set the third argument as false. Thus you shouldn’t use the magic functions, but rather call get_post_meta yourself.

So, just use :

$topics = get_post_meta( $post->ID, 'related_topics', false);

like that you have an array of values