"Margin of error of 9" doesn't mean "It could be [reported result-9] just as it could be [reported result+9]." It just means there's a 95% probability that the result will fall somewhere within that range.
It is Plus & minus 9. It does mean that.
What you are talking about is Confidence Interval. Most surveys are conducted with a confidence interval of 95%.
No, it doesn't. The probability that the result is + or -9 is much, much lower than it will be exactly right or + or -1, for example. Every poll can be turned into a probability forecast, which is what 538 essentially does. The actual probability that it is + or -9 or worse is likely around 5%.
