You can right-click on a document and select "overview" then "source" to see the finished page source (with uncached tags unprocessed) but I think it only works if the page is cached and has been visited. View Source is a more reliable view of the finished page. Neither will let you edit anything. TBH, once you get used to working in MODx, you almost never need to look at the source.
Once you have your CSS and template the way you want them, you only need to edit the resource fields of the document and any chunks or snippets on it. You can also open the CSS file in a separate browser window via cPanel. Many of us install XAMPP and work on the site locally so you can have the CSS file open in your favorite editor. You can also have snippets "include" the source code file and work on that in your favorite editor too.
Note also that if you turn off Rich Text Editor for a page (or for the site), you’ll see the straight HTML code for whatever is in the content field when you edit a document in the Manager.
I agree that MODx is much more robust and has infinitely cleaner architecture than Drupal or Joomla (especially true of MODx Revolution, which is OOP from the ground up).
I’ve used WordPress (though I’m no expert at it) and I would used it again for a site that was mostly a blog site. For anything else, I can work much faster and produce a site that is infinitely easier to maintain in MODx. YMMV.