How to set paragraph margin when editing in CKeditor?
In your example you have:

&lt;p class="rtecenter"&gt;&lt;span style="margin-top:0"&gt;...&lt;/style&gt;&lt;/p&gt;


I'm assuming that should be:

&lt;p class="rtecenter"&gt;&lt;span style="margin-top:0"&gt;...&lt;/span&gt;&lt;/p&gt;


In general in terms of controlling the space between paragraphs I think the best way to do that is your CSS stylesheet, not really HTML. 

Drupal and CKEditor do some HTML manipulation that you may want to checkout - admin/config/content/ckeditor, then choose your profile and click edit.
Under appearance you can select/deselect some plugins that you might not need.

In terms of a quick fix though I would think that this would work:

&lt;p class="rtecenter" style="margin-top:0; text-align:center;"&gt;...&lt;/p&gt;


But it also depends on what is in your site's stylesheet and your theme.