FeedAPI Mapper 2 compatibility

A) - yes, only the functionality of feedapi_mapper_nodeapi() should be moved - not the hook implementation itself (A would remove hook_nodeapi(), C would add the functionality currently done by the hook_nodeapi() implementatin in feedapi_mapper into feedapi_node)
B) This is looking good. I would change the comment to "// Delegate to feedapi_mapper if available". We will need to think how we should implement the delegation of determining the uniqueness of an item in feedapi_mapper: I opened an issue on for that: #539752: Support 'unique' conditions.

Created an issue for A on the FEMP issue tracker #539758: FeedAPI Node compatibility

You can simplify the if/else statement in _feedapi_unique():

if (function_exists('feedapi_mapper_unique')) {
  $feed_node = node_load($feed_nid);
  $dup_items = feedapi_mapper_unique($feed_node, $feed_item);
  if (empty($dup_items)) {
    return TRUE;
  }
  elseif ($settings['x_dedupe']) {
    foreach ($dup_items as $id) {
      $feed_item->feedapi_node->duplicates[$id] = $id;
    }
    return array_pop($dup_items);
  }
  else {
    return array_pop($dup_items);
  }
}

Can be:

if (function_exists('feedapi_mapper_unique')) {
  $feed_node = node_load($feed_nid);
  $dup_items = feedapi_mapper_unique($feed_node, $feed_item);
  if (empty($dup_items)) {
    return TRUE;
  }
  elseif ($settings['x_dedupe']) {
    foreach ($dup_items as $id) {
      $feed_item->feedapi_node->duplicates[$id] = $id;
    }
  }
  return array_pop($dup_items);
}

For the tests: it should be easy to supply a mapping for feed content type by simply creating the proper variable. This can be a stop gap until #433388: Make mappings exportable with CTools/export lands.

This will need adjustments once #539752-8: Support 'unique' conditions is fixed.

2 minor nit picks:

1 This looks a little convoluted:

+  // Let the mapper hijack the decision
+  if (function_exists('feedapi_mapper_unique')) {
+    $feed_node = node_load($feed_nid);
+    $result = feedapi_mapper_unique($feed_node, 'feedapi_node', $feed_item);
+    if ($result === FALSE) {
+      // No user-defined mapping, use the built-in mapping logic (url with guid fallback)
+      unset($result);
+    }
+  }
+  if (isset($result)) {
+    if (empty($result)) {
+      return TRUE;
+    }

I would change it to:

+  // Call feedapi_mapper_unique() if available and use its results if unique elements are defined.
+  if (function_exists('feedapi_mapper_unique')) {
+    $feed_node = node_load($feed_nid);
+    $result = feedapi_mapper_unique($feed_node, 'feedapi_node', $feed_item);
+
+    if ($result !== FALSE) {
+      if (empty($result)) {
+        return TRUE;
+      }

2 Before the original code continues with "if (empty($feed_item->options->original_url)) {" let's add the comment "// If we reach this point feedapi_mapper_unique() is not available or no unique elements have been defined - fall back to URL/GUID for determining whether item is unique.

#12 is a wrong patch.

After this patch feedapi_node should be compatible with feedapi_mapper 1.x and 2.x. We don't need to branch a FeedAPI 2.x therefore - correct?

After #11 is fixed and 1.7 is tagged, this is RTBC.

Could you quickly fix the comment to:

// Use feedapi_mapper_map() if FeedAPI Mapper is version 2.x (= feedapi_mapper_unique() is present)