Joseph has answered your first question. I will go further and suggest that if the journal accepts LaTeX files, do not create a ConTeXt style for that journal. Submit your final source file in LaTeX.
You have to keep the journal work flow in mind. They are not going to experiment with ConTeXt for a single article. Sad but true; it is just too costly. So, if you send them a ConTeXt file, they will simply re-key the whole thing in LaTeX from the pdf. It is better that you re-key it in LaTeX and submit the LaTeX file to the journal. I often do that, and converting the contents from ConTeXt to LaTeX (and verifying the result) usually takes half an hour to two hours.
Regarding your second question, I think that ConTeXt has some advantages if you are working with multiple output formats or conditional compilations. You can do that in LaTeX, but the ConTeXt interface is much cleaner and much more convenient. ConTeXt is also useful for some specialized tasks. For example, ConTeXt can generate pdf from XML source; ConTeXt can generate PDF/A output; for presentations, ConTeXt makes cutting-and-pasting material much more easier (using buffers); for questions and answers, you can type the answers with the questions, but have them appear at the end of a chapter (using blocks). But all these are specialized tasks. (To be fair, there are other specialized tasks for which LaTeX is better).
For simple texts, if you are not interested in developing your own document layout, ConTeXt does not offer any advantage other than a consistent syntax.