If you view the source of your blog in a browser and compare that to the _main.php file you'll see that they differ quite a bit. Because b2e uses php to insert a lot of the information on a blog page you really can't get to it all using Dreamweaver's design section. But some of the stuff you certainly can mess with. Just be sure to use your skin's css file to also make things the way you like.

Nope, but try it out and you'll see.

Basically, php is a server-side language that writes html pages. Dreamweaver is a client-side tool that writes html pages, especially in design mode. You can use dw to edit php files and many people do, but you can't do it in design mode.

Take some time to tinker with your skins/skinname/_main.php file. I got this gut feel that says anyone smart enough to pick b2evolution as their blog engine is smart enough to figure out how to tweak a new-to-them programming language. Look through the code and check out the comments. Comments in php are either long blocks like this:

<?php 
if( $foo ) {
bar();
}
/** this is a
 * comment
 * that takes
 * more than
 * one line
 */
?>

or short bits that look like this:

<?php 
if( $foo ) { // this is a little comment
bar();
}
?>

I still use the split mode (so I can easily see which part of the page I'm editing), and it's how I learnt html. When you use the design pane, look at what changes are made in the code pane. You'll pick it up quickly, if you're willing to learn.

But of course this only applies to the html parts of the file, the php sections only appear as a placeholder.