You can insert JavaScript into your template files but you will need to wrap literal tags around the javascript code. For example:

{literal}
<script language="javascript">
javasrcript code here
</script>
{/literal}

This same method will work for including Google Adsense, Yahoo Publisher, and most other advertising programs.

Yes you can. Just open the template file layout.tpl and add the following code to the top:

{debug}

This popup a javascript debug console and list all the available variables.