Yes, usually. If the privacy settings of the story allow it, comments are visible. But sometimes, if the person who posted the story has some special settings, like hiding comments from certain people or disabling comments altogether, then you might not be able to see them. For example, if they've set their story to be visible only to a specific group of friends and disabled comments for others outside that group.
Sure. If the story owner has not restricted the ability to reply, you can easily reply to comments on FB stories. You'll find an option to type your reply right below the comment. It helps in continuing the conversation related to the story.
Yes, it is. On the Facebook mobile app, you can use the same methods as on the desktop to some extent. Check your profile, look for the Memories tab if available, and use the search function. The app layout might be a bit different, but the general principles apply.
No. When you block someone on Facebook, they generally cannot see your stories. Blocking restricts their access to most of your content, including your stories.
It depends on the settings of the platform or website where the stories are posted. Usually, the author of the story and anyone with the appropriate permissions or access can see the comments.