(A) how do I remove the lines between the title/body and body/tags and (B) how do I add a line/border between posts instead?

To remove the border between the post title and post content and add a border to the bottom of posts in the Forever theme, add this to your Appearance → Custom Design → CSS editor:

.entry-header .entry-meta {
	border-bottom: none;
}

.post {
	border-bottom: 1px solid #EEE;
}

1. I'd like the horizontal border that runs between the main content and widgets to extend to the bottom of the page. Is there a CSS code that will do that?

You could put the border on the right of the content area instead of the left of the sidebar with the following CSS, but note that it may look funny in cases where the sidebar is longer than the post.

#content {
	padding-right: 4.8%;
	margin-right: 29.583333333333%;
	border-right: 1px solid #EEE;
}

#secondary {
	border-left: none;
}

1. I'd like the horizontal border that runs between the main content and widgets to extend to the bottom of the page. Is there a CSS code that will do that?

2. On the main page, the posts have lines between the title and main body text and between the main body text and tags. However, there is no line between the posts themselves. I think it looks a little confusing. See what I'm talking about here: http://atthetreehou.se My questions are: (A) how do I remove the lines between the title/body and body/tags and (B) how do I add a line/border between posts instead?

Thank you!