Vince,

Welcome to Drupal! I'll try to answer each of questions as best I can in the limit space of a comment.

1. If you leave in the website field that is exactly how it works by default. The username links to that value.

2. The same way I removed the homepage value in the example.
$form['subject'] = NULL;

3. I beleive you are looking for the comment notify module. Be careful though since it is currently an alpha version for D7.

4. We are not using a module for the social sharing buttons. We just took the code from each service (i.e. Facebook Like Button) and pasted it into the proper template file (example: node--blog.tpl.php). I will say that it's hard to get all those buggers to line up right since some of them are iframes and they aren't uniform sizes. I'd love to see some standardization of these buttons across services, but that's unlikely.