If you are asking can you use an existing XML element (and all of its children) to append or insert somewhere else in the XML structure, then yes. Instead of dynamically building the structure, you can just select the single node, clone it, modify it as necessary, and then append or insert it the same way as in the example. The key is that you need to clone the node. You can't use one object and try to append it in several places.
I think I've mention this to you before in a previous post. Personally, I don't use the XML UDF. I guess for some it may make some tasks a little easier but it is missing some functionality, has a few bugs/issues (some I have pointed out years ago that still are not fixed), and for the most part, just adds a lot of unnecessary overhead to the use of the COM objects themselves.
Obviously, _XML_Tidy() was not necessary. I only used it to make the output more easily readable for anyone who ran the script. There are plenty of ways to pretty-print XML. I prefer to use a much more feature-rich command line utility (xmlstarlet) to do my XML formatting.
As far as site/documentation that will help you learn how to use and implement XML DOM methods and properties, I would start at the source, MS XML DOM Reference. Another very good site for all things WWW, is W3Schools Online Web Tutorials and References. In terms of XML-related information, here is the link to the main XML page. There you can learn about XML in general, the XML DOM and how to navigate it, XPath, XSLT, and much more. Plus, many of the tutorials allow you to interactively try and test the information you are learning. It is a VERY good site for learning W3 stuff.
On a side note as it relates to KML styles, are you aware that you can define the styles once and reference them in your Placemarks by using a <styleUrl> tag? Defining the styles once makes it much easier to maintain the KML files if you need or want to change a style later. Creating duplicate <style> nodes throughout a document is a maintenance nightmare. Here is a sample on the Google Developer's Site that shows what I mean.