I will answer the points in order:
1) I think it would be unreasonable to pick a particular difficulty of class and say that everything under that level is off-limits. It seems too arbitrary. I think a good benchmark for whether or not a question is appropriate should be gauged by the amount of effort the person has put into asking the question. If asking a question requires the person to put REAL work into asking, this will deter many people from just dumping their HW on here. For instance, a college algebra student doesn't know how to factor a cubic. If that student says "I have tried rational coefficients, and I got blah blah, I tried guessing a root to reduce down to a quadratic with long division but couldn't find a root blah blah. I also did blah blah" Then clearly the student has put some thought into the problem. This is what I would consider the absolute minimum required to help them. By encouraging them to think for themselves and ask robust questions, we will deter the kind of questions we don't want, and encourage those we do.
2) I think the answer should be very dependent on the style of question asked. If it is clear the question is not HW, answer in great detail. If there is a chance that it is HW, attempt to troubleshoot the issue they are having. Identify where they are going wrong/failing to see the idea and address that. Again, make them work a little for their info. If they really care about the answer, this will be worth it to them.
3) If it is impossible to address the question without a complete solution, or the solution is so trivial that it fits in a comment box, then why not just comment? The reputation for answering these questions shouldn't motivate you that much. Everyone should be their own judge if they "deserve" rep for an answer. Links to answers should always be comments, save the case when the question was a reference-request.
4) If they are doing self-study, they should be more than capable to make their question convey that they have put some effort in. Additionally, if they are motivated to do this self-study, they will be motivated to make their question better and get more interesting answers.
5) See 1)