Database schema of comments
Yes.
Combined with 'entity_type' = 'comment', yes. An entity (comment, node, term, user, ..) is always identified by their entity_type and entity_id.
Not directly, in this case. There is also field_revision_comment_body table, which is in this case not used because comments don't support revisions (but field always needs to create one because nothing stops you from adding that field to a node and create revisions of it). There are also the field_config and field_instance_config tables which contain information about which field is attached to which entity type/bundle.