How do accept a comment as the correct answer to my question? As a newbie, I can't vote, and existing posts about this issue tell me I should click on the grayed out check mark next to the answer. But in the case of comments there's no such grayed out check mark. And if I 'Answer my own question', I would merely be repeating what the comment says.
Suppose an answer (or comment!) offers a partial answer, but I want to continue the dialog, saying 'yes, but...'. Or suppose an offered answer (or comment!) simply didn't work and I want to report that fact? To the extent I've understood your etiquette so far, I should do so by editing my question itself. Is that right? So should I start this by saying something like 'According to one of the comments below...'?
Leave a comment requesting the user convert it to an answer. See Why do people answer in comments? and Adding the "accepted" mark to a question or to a comment.
Typically a "discussion" ensues within comments to the answer. This brings attention to the answerer to possibly update it to suit the new requirements. On the other hand. If the question is solved in this way, some users tend to leave their original question as-is. Others tend to feel that a modified question more adequately reflects their (original) intent, and therefore do exactly that.
I always think that it's best to leave the post in a very general format, as it serves a purpose on its own. Explicitly stating "Edit 1" or "Edit 2", or mentioning that "some new request is as a result of some comment" makes people have to find and read the related comments to fully understand what went on. And that isn't always necessary in my opinion. You can identify post edits by looking at the revision history, and comments vary in their usefulness and shouldn't always be referred to as being part of a question/answer.
What is important though is asking a good question to start with. Yes, this is not always easy. Do your research before "throwing in the towel"... One thing that has shown to help time-and-time again is the construction of a minimal (working) example (or MWE). Very often, when breaking down a problem into its minimal components, it reveals clues as to what the solution might hold. Moreover, it shows that you care and would serve the community a problem on a silver platter... rather than just dumping your question using a fix-this-for-me approach. Never under-estimate the power of a minimal (working) example.