ELL is a StackExchange site and whilst we can be a little different from other sites we are still fundamentally part of the SE network. We can still foster a positive learning culture here without the need to break all of the SE "norms" and Meta is the perfect place for us to discuss these issues.
I'll address each of your points individually:
Absolutely no sarcasm or any other kind of condescending tone should be tolerated in answers. Possibly, sarcasm/condescension would be lost on some lower-level learners, but it has absolutely no place in this environment. Responders who continually answer questions in such an inappropriate manner should initially be warned, and, if such egregious behavior continues, banned from the site.
I haven't personally seen this yet on ELL, but please feel free to link to posts where you believe this is happening on here. If you have an issue with the content of a question and/or answer, you are free to edit the content (as long as you don't change the original meaning) to what you consider an accepted standard. Stack Exchange sites are community edited, moderated and contributed to.
I would also like to point out here that these sorts of comments are not appropriate and don't help you argue your case. As I've mentioned in comments to you before, Meta is the appropriate place for these sorts of discussions.
Whilst we are in beta we're still shaping the site - and you can help us doing that by continuing to provide good answers (like you have) and raising issues on Meta for the community to discuss and ultimately resolve.
Responders must grade their language down to a level appropriate for/accessible to an ESL/EFL learner. Answers which are insufficiently-graded should be edited by others.
I agree with this and it has also been discussed here on Meta too. If you feel an answer is too technical for the target audience, feel free to post your own or edit the answer to improve it. The best answers will be upvoted to the top.
New questions which duplicate previously-asked questions should not be closed for that reason, EVER. Many lower-level ESL/EFL learners do not possess sufficient English language skills to determine this beforehand. Of course, responders can link to previous answers.
Not many questions are closed here as yet, but as @StoneyB mentioned - it isn't an unfriendly act and can help the asker find an answer to their question without the need to duplicate content that is already available. If the answer on the "duplicate" isn't what the asker is after, this will assist them to target their question further.
If a new question is thought to be too basic for the forum because the answer could be found in a dictionary or other readily-accessible source, such a determination (for example, to close the question) should only be made by certified ESL/EFL instructors. I have seen too many great questions shut down on ELL by responders who obviously have never taught in an ESL/EFL environment. (No doubt, such a change would require an assessment or ranking of responders that would be onerous; as an alternative, I would instead simply suggest that no new question that is thought to be too basic be closed, EVER. See #3.)
I wholeheartedly disagree with this statement. A question closure isn't binding, and we can always reopen a question based on community discussion (which has happened with some questions already). Just because someone hasn't taught in an ESL/EFL environment before doesn't mean they also can't provide quality content to our community or assist in the moderation of it. Certification as an ESL/EFL teacher is different in every region (e.g. it's a 2 day course here) and we shouldn't police our content and community based on what their qualifications (if any) are. As I've mentioned, SE sites are moderated by the community and when a question is closed and you don't think it should be - raise it on Meta.
All responders must consider that 99% of the ESL/EFL learners who are posing questions on ELL are sincere and eager to learn. They should always, always, always be treated with respect and friendliness.
Absolutely. The other members of this community should also be treated with respect and friendliness.
Completely do away with the silly prohibition on friendly expressions such as "Hi, [X]", "Thank you for the question," and "I hope this helps." In an ESL/EFL environment (and, I would argue, in any other kind of environment, but that is an argument for another day), friendly expressions should not be edited out, EVER.
SE sites are not forums, we work a little differently here. Salutations and the like add clutter to a question/answer and the SE sites serve more than just the primary author of the question.
SE even automatically removes salutations from posts as they are entered.
When a post is edited to remove these, please do not roll it back - as we want to avoid "edit wars". After too many edits your answer will become Community Wiki automatically, which means you'll miss out on any Rep gained from Upvotes, and a much lower reputation threshold (100 rep) is needed to edit the content.