May I ask a slightly daft question: I assume one would use the "Add article" link, on the top right of the page, on the articles section, to write an article? Does the "Save" button save a draft or actually publish the article, in other words is it possible to write an article over a period of time in several drafts/passes?
Every article is published only after proofreading by moderators. Sure, an author can sace as many versions of his artcile as needs.
What does the review process entail to be eligible for the advertised author's fee? Is the author's fee a "one-time-only" offer, or per article? Assuming submission of good quality articles on new subjects, is there a limit to how many articles one author can write in (i) a given time period, or (ii) in total, and still be eligible for the author's fee?
No any limits on quality of articles, see for example this series. We pay for every article if it is interesting and meets our standards. See also New Article Publishing System at the MQL5.community
Are there any restrictions on subject matter at all, or is adopting a "common sense" approach sufficient?
The article for publishing must have source codes and examples of trade strategy, EA and etc. We don't publish general articles.
Please do not make an empty quotation.
Can the article simply have all the source published on GitHub? I have an open source unit testing library which is quite involved.
Certainly, one can have source files published anywhere. But all files for article must be attached there - on this site.
I'm chinese,I wrote a lot of Chinese articles about mql5 on myself website http://www.mt5.net.cn/.
I very like to become your author and publish my articles on this website in English in the future.
OK, welcome. But let me see your first article in the draft, I have to check its subject
why cannot add an Mql4 article?
I cannot chose MQL4 tab
It's not available anymore. If your article is about MT4 currently, consider re-writing it in a cross-platform way, that is suitable to both MT4 and MT5, then apply under MT5.