This is how I do it (IIRC, this is because different versions of Umbraco store things differently):

https://github.com/rhythmagency/rhythm.umbraco.extensions/blob/e3cce1803863d452dce76784ad554ddb6529dce0/trunk/Rhythm.Extensions/Rhythm.Extensions/ExtensionMethods/PublishedContentExtensionMethods.cs#L213

And here's a snapshot of that code snippet for your reference:

/// <summary>
/// Gets the ID's of the picked nodes.
/// </summary>
/// <param name="source">The node with the picker property.</param>
/// <param name="propertyAlias">The alias of the picker property.</param>
/// <param name="recursive">Recursively check ancestors (false by default)?</param>
/// <returns>
/// The picked node ID's.
/// </returns>
/// <remarks>
/// This is faster than LocalizedGetPickedNodes when you only need node ID's.
/// </remarks>
public static IEnumerable<int> LocalizedGetPickedNodeIds(this IPublishedContent source,
    string propertyAlias, bool recursive = false)
{
    if (recursive) {
        while (source != null) {
            if (source.HasValue(propertyAlias, false)) {
                break;
            }
            source = source.Parent;
        }
    }
    if (source != null) {
        string pickerValue = source.LocalizedPropertyValue<string>(propertyAlias);
        if (pickerValue != null)
        {
            int nodeId;

            // Integer, CSV, or XML?
            if (int.TryParse(pickerValue, out nodeId)) {
                yield return nodeId;
            }
            else if (CsvRegex.IsMatch(pickerValue))
            {
                var pickedNodes = StringUtility.SplitCsv(pickerValue);
                foreach (var nodeItem in pickedNodes)
                {
                    nodeId = int.Parse(nodeItem.Trim());
                    yield return nodeId;
                }
            }
            else if (!string.IsNullOrWhiteSpace(pickerValue as string)) {
                var pickedNodes = new DynamicXml(pickerValue as string);
                foreach (dynamic nodeItem in pickedNodes) {
                    nodeId = int.Parse(nodeItem.InnerText);
                    yield return nodeId;
                }
            }

        }
    }
}

ok, well still more complicated than I was hoping.

I trimmed down your example and wrote a very short method that works for me in this case.

    /// <summary>
    /// Gets the ID's from data type content picker stored as XML
    /// </summary>
    /// <param name="source">The node with the picker property.</param>
    /// <param name="propertyAlias">The alias of the picker property.</param>
    /// <returns>
    /// The picked node ID's.
    /// </returns>
    public static IEnumerable<int> getContentPickerIDs(IPublishedContent source, string propertyAlias) {
        if (source != null) {
            var pickerValue = source.GetProperty(propertyAlias).DataValue.ToString();
            if (pickerValue != null) {
                var pickedNodes = new DynamicXml(pickerValue);
                foreach (dynamic nodeItem in pickedNodes) {
                    var nodeId = int.Parse(nodeItem.InnerText);
                    yield return nodeId;
                }
            }
        }
    }

Thanks for the quick response!

is working on a reply...