How to 'inject' the "read more" link into a textfield / paragraph (inline)
If the goal is simply to get the read more link to appear visually inline with your last paragraph, I think you can do that with pure CSS.  As you noted, formatting all your &lt;p&gt; tags as inline elements will break the paragraph structure.  You can avoid this simply by using a :last-child selector, e.g. http://jsfiddle.net/hdys6w2j/

That said, if you are set on having the readmore link injected into your last p tag, I guess you could throw some code like this in your custom module:

function mymodule_preprocess_node(&amp;$vars) {

  //Programmatically generate the readmore link
  $read_more_link = l('Read more', 'node' . $vars['node']-&gt;nid);

  if (!empty($vars['content']['my_field'][0]['#markup'])) {

    //Replace the last &lt;/p&gt; tag with &lt;a href="/path/to/my/node"&gt;Read More&lt;/a&gt;&lt;/p&gt;
    $vars['content']['my_field'][0]['#markup'] = preg_replace(
       '/\&lt;\/p\&gt;$/', 
       $read_more_link . '&lt;/p&gt;',
       $vars['content']['my_field'][0]['#markup']
    );
  }

}