In answer to your points - 
 *1. United Kingdom is used because it is the most common name for current country (same way the United States article is shortened from United States of America. If there was another change and another new United Kingdom was formed then that would be called simply United Kingdom and the current state moved to UK of GB & NI. 
 *2. Most european articles predate the actual state described. If you apply the same logic to the Germany article then it should only mention events after 1990 or West/East/Nazi Germany articles should all be scraped. This article deals with a general overview of the entire history of the region. This could probably called the 'parent' article to all UK based topics but also acts as the main article for the current state. 
 *3. It was formed in 1922 but the name UK of GB & NI wasn't adopted until 1927 
 *4. It doubles up as both the article for the current state and the parent article for all countries that exist/existed within it.  
 *5. As I explained it's a abreviation 
 *6. We need a central article to explain the country called the UK of GB & NI but there is no way that it can be used to cover the entire history of the country. As you'll see by the diagrams below there has been 8-11 countries that have made up what is called the UK (depending on definition and not including mercia etc). Most of these have substantial articles themselves. You just happened to find the smallest one and decided it needs merging. 

 United Kingdom of Great Britain and Ireland has been expanded since this debate started and there isn't as much overlap. So to merge in the article (which still needs improvement) would create more problems than solve. I don't see what your problem is, are the servers struggling to hold that extra article. We need to increase the number of articles the History of the United Kingdom is 78K this one struggles to stay a reasonable size.