But in English "advice" is uncountable, so it covers more than a single piece of advice. Therefore, Duolingo ought to accept it.
(This is similar to the "matunda = fruit/fruits" questions: just because matunda is a plural noun in Swahili doesn't mean we should avoid the correct translation into the English uncountable noun "fruit". If we feel a need to indicate a plural, we say "pieces of fruit".)
Yes, I agree with you. "mashauri" means advice, and it is used when you ask for advice or you want a second opinion on any matter, since in swahili they mean to address every single person for their advice they use the plural form. If you want the advice of one person only you will say "Shauri yako".
"Tunda" is singular for "fruit" in the sense of one tangible piece of fruit/a single tangible fruit. "Matunda" is the plural form for more than one tangible fruit. BUT, "matunda" is also the term for the general concept "fruit", just like "mboga" is for "vegetable". It's like "person" and "people". "Person" means a singular person--only one of many people, whereas "people" is one of two possible plurals for person: "persons" and "people". But "people" has two meanings: one is several concrete/tangible persons/people, and the other is the general concept "people" as also meaning human beings.
I agree. I reported it today, 28 Feb 2018. (I am curious to see how long it takes to see a beta report fixed, but I suspect it is a very long wait. When viewing the comments in the app, the date, e.g. "9 months ago", isn't shown - that's why I'm adding it in my comment.)
There are many possible English translations -- your affairs, your concerns, your business, your matters, your issues, your problems, your predicaments -- and others. I am not sure if mashauri can be used to mean "the plural of advice". I don't think I have seen it used in that way -- rather always in the singular -- shauri.