It looks like you copied the entire twenty eleven style sheet into your child theme style sheet. You don't need to do that. Your child theme style.css required only the header text and the changes you would like incorporated into your theme.
On to your questions...
#2 - You need to make two changes
.hentry, .no-results {border-bottom: 0px solid #DDDDDD;}
and
.one-column #nav-below {border-bottom: 1px solid #DDDDDD;}
#3 - copy content.php in the twentyeleven folder into your child theme folder.
Near line 77 there is a line that starts with <span class="comments-link">
Wrap that line in paragraph tags, so it looks like this:
<p>
<span class="comments-link"><?php comments_popup_link( '<span class="leave-reply">' . __( 'Leave a reply', 'twentyeleven' ) . '</span>', __( '<b>1</b> Reply', 'twentyeleven' ), __( '<b>%</b> Replies', 'twentyeleven' ) ); ?></span>
</p>