Earlier quoted context omitted.
Fluent with Duolingo alone? Very unlikely in my experience. From my point of view, you can make it to A1/A2 proficiency in a language with Duolingo alone - after that, it gets quite hard. Teaching grammar and conjugations isn't its forté, without resources outside of Duolingo it will get rather tricky. That being said, I still use Duolingo for my daily dose of language exposure - but without Anki (for vocabulary), yo…
I'm using Duolingo to learn Russian because my spouse is Russian and I want to communicate better with his family (especially his grandmother, who speaks less English than I do Russian). After nearly a year, I can help set up for dinner ("Where are the plates?" "There is bread on the table already" "Do you want tea?") and similar things. Now I'm getting into genitives and such, and that's where Duolingo really seems…
Most native speakers are useless for why. You ideally want someone who learned both your native language and the language in question. Then they can often explain things relative to your language, and you'll learn things about your language and the language in question. Look for language notes from Hong Kong; I had found a university site with very helpful information on French many moons ago, but I can't find it again, it was written in fluent English, but with insights on English usage that were unlikely to occur to native English-only speakers.