I've asked a couple of questions that have a number of answers that are useful but seem to miss the main point, even those that have been added after extensive editing aimed at clarification of the area of interest. Many of the answers indicate to me that the person answering sees what I'm getting at and yet somehow manages to contribute an edge of the picture while missing the centre. I'm reading 5 or 6 answers that do this at a time, plus comments that give me other bits of the picture, without getting an answer to the question as a whole. It feels like I'm getting nowhere even when I can see I am, the frustration is getting to the point where I look longer and longer at the Delete button every time I visit the question. Is this a common issue and if so how do people cope with it?
It is a common issue
This issue comes open more often than you may think. Two recent examples of people with this issue who started Meta discussion were Vylix, who started the discussion Asked a bad question, can't delete, can't close by myself. What is the correct thing to do? and adaliabooks, who started the Meta discussion Is it better to accept an answer that's not quite what you were looking for or leave a question unanswered?.
As you can see those two had a very similar problem and both had a different view on this: adaliabooks was mainly concerned with the question whether you should accept an answer or not if it's not quite what you were hoping for, while Vylix was asking about whether he should delete a question that was not framed in a way that he would get the desired answers. Both of these cases are similar to your more general case of being frustrated because answerers are getting lost on tangents.
There are different strategies to focus the attention of answerers
In both of the questions I referred to earlier the community tried to give feedback by referring to some of the tools available on WorldBuilding to prevent or at least mitigate this issue.
You already mention edits. Just for completeness sake I will mention that editing your question and commenting on the existing answer what exactly they are missing should be the first step when you realize that people are going off a tangent. At least if you don't invalidate answers. Editing should happen as fast as possible, before others jump to answering a not-quite-right question. That's why it's important to stay online at least a bit to check whether people are requesting clarification after you posted your answer.
The next one is our Sandbox. The Sandbox is a great place if people have told you that you need to format your question, work on your titles, need to give more or less information in your questions and all around left a lot of comments about the way you write questions on this site. We recommend the Sandbox especially to users who show problems, but it is used by experienced users, too. It's a place to get feedback on all this general stuff before posting it on the Main Site. Some people use it because they know they need some help with their english from time to time. Others use it when they know a question might be borderline "Too Broad" or "Too Opinion-Based". Others use it just to be sure.
In any case: this is a great environment to get tips on the style first to be sure people focus on the real question asked. On the Main Site you have quite a few people who will answer everything on sight as soon as you post it. This might make it more difficult to later edit the question without invalidating answers. By having others look over your draft you can make check whether people understand what you are looking for or not and fix those problems before other people jump to answering.
The third one I want to mention is to use the bounty. After two days you can place a bounty on any question. There you have different pre-defined options like "This question hasn't received enough attention" or "The answers need more detail". Then you can specify a specific message to show what exactly you are missing. If this is additional information this tactic is perfectly valid and often attracts a few high-quality answers as people are looking forward to the special reward.
The fourth one is to ask a new question. In this question you should refer to the old one in the first paragraph and mention explicitly what is different and would invalidate answer in the other question so that people focus on this aspect and won't vote to close your new question as a duplicate. The Sandbox might be useful to make sure of this, but in general this is the preferred method if your edits would invalidate existing answers.
The fifth one is the chat. You can get quick feedback on drafts in the Sandbox or ideas in the chat. You can even talk about your question and brainstorm there to see if the people in the chat can give you more detailed information.
There are different strategies for coping with this issue
You can repeatedly ask a question with a slightly different premise. You can make sure by checking in the Sandbox for a week and asking in chat for feedback. You can edit as fast as you can and hope you get the right answer. You can wait and use bounties to get more focused answers after a while.
It totally depends on you. Everyone copes differently with this problem. Some people don't even see this as a problem and are glad for a different perspective - and the chance to post another question and get more reputation from the whole process.
Tips from what I see in this Meta post
Your title doesn't seem to match the content in this case. I was 100% sure that you were going to rage quit the site after getting a lot of bad answers to your questions until I was at your second to last sentence at (emphasis mine):
Delete button every time I visit the question.
That was the first time I realized that you want to talk about something similar to Vylix and adaliabooks and also why I at first wrote comments asking for clarification. Maybe you could try to focus the question title more on the real question you want to ask and word it like a question. For example
How do people cope with answers that go off on tangents?
The next problem I see is that you just put a big block of text into your question. By just using a paragraph, meaning two linebreaks, between your introduction and the real question you can focus the attention on the most important things. The best thing would be to ask the real question at the beginning, but it's also common to write it at the end like you did. People often only read the first and last paragraph. And if there is no second paragraph some people tend to read the title, the first sentence and the last sentence. This sometimes leads to weird answers, because people don't read the whole question body. And sometimes this leads others to only read the existing answers and going even farther off.
One of our mods wrote a great series. I recommend you give it a try: Lessons in writing questions. He mentions a lot of very interesting points that are specific to the StackExchange format and WorldBuilding. Therefore it's a great read if you think people are having trouble understanding what you are really asking.
Don't give up.
You're not alone.
This is a common issue and it happens to everyone at some point. Some people see this more often than others and everyone handles it differently. If you haven't found the answer you were searching for there are always options to get them. And the community will do everything they can to help you get those answers.