Customization

For the first problem, go to Appearance → Customize → Additional CSS and add this rule:

@media screen and (min-width: 61.5625em) {
	body.blog article.post div.entry-content {
		float: none;
		width: 100%;
	}
}

For the second problem, I don’t know what the cause is, and it seems to only happen with the first post, i.e., the second & third posts don’t seem to have the extra space between the post title and the excerpt.