[SalesForce] Parse dynamic JSON with Apex

I am receiving the following JSON as a result of a web callout:

{
  "count" : 1,
  "posts" : {
    "1234567" : {
      "id" : "1234567",
      "private" : false,
      "message" : "0D5j0000004bV8oCAE - Salesforce Chatter - New post on Monday Jan 5th",
      "subject_ref" : null,
      "created_at" : "2015-01-05T09:56:59-08:00",
      "story_id" : null,
      "newest_reply_at" : null,
      "reply" : false,
      "subject_type" : null,
      "subject_id" : null,
      "user_id" : "09876543",
      "updated_at" : "2015-01-05T09:56:59-08:00",
      "workspace_id" : "4569872",
      "has_attachments" : false,
      "reply_count" : 0,
      "formatted_message" : "<p>0D5j0000004bV8oCAE - Salesforce Chatter - New post on Monday Jan 5th</p>"
    }
  },
  "results" : [
    {
      "id" : "1234567",
      "key" : "posts"
    }
  ]
}

And I am making use of JSON.deserializeUntyped as follows:

Map<String,Object> m = (Map<String,Object>) JSON.deserializeUntyped(res.getBody());
Map<String,Object> posts = (Map<String,Object>)m.get('posts');

List<String> return_posts = new List<String>();

for (String p : posts.keySet()) {
    List<String>  pp = (List<String>)posts.get(p);
    return_posts.add(pp);
}

However I am running into a couple of issues:

1. "Incompatible element type LIST for collection of String when trying to add the posts array to return_posts inside the loop.

2. If I comment out the line causing error #1 above, the code compiles but throws this error at runtime:

   FATAL_ERROR System.TypeException: Invalid conversion from runtime type MAP to LIST

My goal is to create a list/array with jus the post content, similar to this:

{
    "id" : "1234567",
    "private" : false,
    "message" : "message body"
    etc....
},
{
    "id" : "0000000",
    "private" : false,
    "message" : "some more message body"
    etc....
},

There appears to be some type mismatching going on, and I may be missing one loop level to get to the "posts" level, but I'm having difficulty getting to the root of the issue. Any help would be really appreciated. Thanks!