Originally Posted by why
'Commonwealth English' is known as Received Pronunciation (usually stated as RP), while 'American English' is commonly referred to as General American (GA). Americanisms are not really part of RP or GA since they're usually morphological.
No, RP is a very specific accent that is basically unused nowadays. Even the Queen had partially drifted away from it. It arose centuries ago from monarchs being tired of not being able to understand those who held court with them from other parts of England. It has nothing whatsoever to do with diction, spelling, grammar, etc...accent only. "Commonwealth English" is a broad term defining shared characteristics among the Commonwealth nations, particulary spelling, and pertaining less to accent, since the Commonwealth nations display different de facto standard accents.
The loss of 'u' in American dipthongs (neighbour, colour, armour, valour, candour, etc.) is partly due to Webster and others who felt America should have a distinct and official language. Some leaders of young America played with the idea as well (there's a document attributed to John Adams where he tries to argue the necessity of an official language) but these ideas obviously never got very far.
Sort of, although these aren't dipthongs, they're diagraphs. Dipthong is a phonetic/phonological term referring to two vowels coalescing and forming the nucleus of a single syllable; diagraph is the use of two letter characters for one sound (like 'th').
Some people, most notably Ben Franklin*, argued for different spellings for patriotic reasons, but Webster's reason was purely practical. He noted that the '-re' spelling of words made sense in French, but due to the pronunciation changes when English borrowed these words the spelling didn't make sense in English. The same more or less goes for most of his reforms, many of which never caught on ('iz' for 'is,' for example).
Intensely interested in language, he was one of the main proponents of (a.) widespread and free education, and (b.) standardization. The main speller being used in the late eighteenth century was Dilworth's speller (don't remember the title), which was unabashedly Torry and consisted of moralistic parables. Webster wanted children to think a little more critically, which caused him to self-publish his "blue-backed speller." This began outselling Dilworth and was, I've argued in research I did years ago, the most influential item in the solidification of American spelling. His dictionary is thought by many as his most influential work, but there's evidence that his spellings were taking hold among the young workforce in cities before the dictionary even appeared.
*Ben Franklin even invented his own alphabet and used it in letters to friends for several years but finally gave up.