Diacritical marks are a way to imply pronunciation in scripts that lack vaowels (Most semeitic languages, i.e. Arabic, Hebrew, Aramaic, Syriac, Coptic, etc...). BTW, how can you have made the comments you have without even being able to read Arabic?
As far as changes in the content of Q'uran over time, changes are typical in any texts over time. I did a seminar paper on dietary injuctions in the Q'uran in the period 900-1100, and used texts that represented this change (gotta love the DC area. Access to Library of Congress, georgetown's Arab Studies Collection, and the Walters (Baltimore).
). There were distinct differences between texts, not only in meaning, but in ordering of Suras. The Q'uran probably didn't acheive its current form before the print era, when it became possible to standardize texts.