Welcome to modx.
1. Modx can be used as per your requirement.
a> It act as framework for the developer.
a.1> you create reusable,pluggable component in form of snippets.
b> it act as cms for end user perspective.
2. my answer to your confusion regarding the snippet that you need to paste every thing in their only.
a> No you never required to paste all the code in the snippet.
b> You can use DW or PDT for developing with modx. my personal preference is the PDT. you can down load from zend’s site.
http://zend.com free of cost. It will enable you to debug and navigate to classes and function , inline tips and many more easily compared to DW.
Oh yes, you can do your css or html page design on DW.
c> the steps to create new snippet will be
c1> create a dir "mynewsnippet" under "assets/snippets/"
c2> create a php file "mynewsnippet.php" under "assets/snippets/mynewsnippet/"
c3> add some php code inside mynewsnippet.php file i.e
<?php echo "hello world"; ?>
c4> now open modx manager in the browser . go to resources->manage resources->snippets
c5> click on new snippet. enter the snippet name "mynewsnippet"
c6> in the " Snippet code (php)" section add following line
<?php include_once($modx->config['base_path'] . "assets/snippets/mynewsnippet/mynewsnippet.php");?>
and save the snippet.
c7> now create a new document and call this snippet by putting this non cacheble call [!mynewsnippet!] in content area and save the document.
Now preview the document by right clicking on the document and selecting the preview option. you will be able to see the hello world page.
if you want to change the hello world output then you just change the code in "assets/snippets/mynewsnippet/mynewsnippet.php" . and it is more better and convinient as it t can be done by your DW or any notepad.
3. regarding the admin side area for your clients.
a> you can enable friendly url and create a document folder called admin and keep all the document their specific to admin part. you can create a user group and and set the access permission to them.
b> you can use modules in modx managers for your task as well for super administrator.