Add this CSS in Appearance > Customize > Additional CSS:
.single-post .site-content {
margin-top: -50px;
background-color: #fff;
}
.single-post .entry-header {
text-align: center;
}
Then, set the Background color of the Container Block holding the Author Image to transparent – Give it 0 opacity.
The CSS above addresses the two issues you mentioned in the Screenshot. The vertical line is from the Site Content. In the code, I placed the site content to -50px from its position.