It is the equivalent to using colloquial English, African American Vernacular English, etc. in terms of English. It is typically not acceptable in books, newspapers and magazines.

1. Yes, however natives use arbitrary spelling and could differ from person to person.
2. In some ways. You may not use it in school, but it is encouraged online. Here are examples using Roman script from online:
https://twitter.com/bakyzle/sta...59747349327872
http://tv.ohkhmer.com/v/818
http://lastlovelesslove.blogspo...1_archive.html
3. It is split right down the middle. Some will say that spoken Khmer should NOT be written down, and that it is a bastardization of standard Khmer. However, few people speak the way that can be represented by Khmer script (example: in writing, the word 'taə'(តើ) is basically required at the beginning of a question, even though NOBODY ever speaks it).
4. It has become a trend to do so, but usually only on the internet and as texts.
5. All Khmers are taught Khmer script in school. I DO know some who did not go to school and learned only the alternative way to text message friends.
6. No, but then again, it is like "street language" in any language, and most teachers would not recommend that.
7. Again, probably not, nothing outside of the internet of course... but then again, it may change someday.
8. Absolutely, there is a great wealth of language (natural, spoken language) that cannot be properly expressed in Khmer script, because the script was not created to encompass it. And again, if I were to try using the Khmer script to write as the language is truly spoken, it would be seen as incorrect. Not allowing the 2 different sets of characters to write Khmer would be like deleting sentences marked as "Non-standard English". We need to allow it in order to hear the true Khmer language, otherwise we end up with unnatural spoken sentences (reminiscent of something like 「私はあなたを愛しています。」 in Japanese, and we all know how much tommy_san hates that!)
The only other solution I can think of would be to link to an audio-only sentence... but that just sounds terrible.