YAML Merge Keys status and implementation

Hello.

We've found YAML library, that we use (in fact we tried two: wrappers
based on libyaml and libsyck) does not support "YAML Merge Keys"
feature (a type).
http://yaml.org/type/merge.html

We've made some fixes to wrappers to support this feature and it
worked so far, but recently we found that

the following YAML:
===
_message_title: &_m_title
  title: sometitle

_message_text: &_m_text
  content: somecontent


items:
-
  fixture:
     <<: *_m_text
     <<: *_m_title
===

Parsed by this online service http://yaml-online-parser.appspot.com/
(I think python library) as:

===
{
  "_message_title": {
    "title": "sometitle"
  },
  "_message_text": {
    "content": "somecontent"
  },
  "items": [
    {
      "fixture": {
        "content": "somecontent",
        "title": "sometitle"
      }
    }
  ]
}

===

and our library+fixes does not support this.
Now we're trying to understand status of Merge Hash draft and level of
support for it, that we should implement..

Questions:

1) I understand that http://yaml.org/type/merge.html is optional, and
a YAML parser is not obligated to support it.
Is this correct? How many YAML implementations support it in the wild, in fact?

2) In above example the following construct (not described in the draft)

(form A)
===
  fixture:
     <<: *_m_text
     <<: *_m_title
===

is parsed by http://yaml-online-parser.appspot.com/. But should it be?

or more correct form would be (and it's described in the draft):

(form B)
===
  fixture:
    <<: [ *_m_text, *_m_title ]
===

Should/MAY/MUST/MAY-NOT YAML parser (which tends to support Merge
Keys) support the first form (form A)?

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