1. Is it ok to write IELTS essays in capital letters?
Yes, it seems that the rules changed some time ago, and it's now fine to write your essays in capital letters.
2. Is writing task 2 the same for the general and academic tests?
Yes, the question types and topics are basically the same. That's why I only have one lesson category for task 2 here on my blog.
3. Do examiners care about small things like singular and plural forms?
Yes, examiners notice these things! In the writing test, one small mistake won't affect your score, but many small mistakes will. In the listening test, a singular / plural mistake could lose you a whole mark.
4. Is it better to write numbers as words or figures?
It's probably better to write the full word (e.g. 'nine') in writing task 2, but I would write the figure (e.g. '9') in all other parts of the test.
5. Is it ok if I speak with an American accent?
Yes, of course! Examiners care about pronunciation and clarity, but not accent.