"My aunt is nice, not strange."
Translation:Mia onklino estas afabla, ne stranga.
afabla = friendly, good-natured, afable
agrabla = pleasant, enjoyable, agreeable
Both are quite similar and can mean "nice".
I'd explain the difference when describing someone as "afabla" being an outward nice or kind, extroverted nice. While "agrabla" is a more passive nice, introverted nice.
I don't believe so. You'd have to reword the second part as well, which would turn it into a separate clause, which I believe requires a connector and its own subject pronoun, which would most likely take it too far away from the original for Duo to still consider it an acceptable translation:
Mia onklino afablas, sed si ne strangas.
(Grammar note: "si" is not a typo; it's the reflexive, which here stands for "she herself".)
To me, it's not a good idea to think too much about verbizing adjectives in Esperanto. I recommend this blog article:
“Ŝi afablas” is not “she is nice” but more like “she (now) behaves nicely”; likewise for “ŝi strangas”. Verbs are not adjectives, at least not in Esperanto. Thinking too much about verbizing is like trying to outsmart the Esperanto grammar instead of making good use of it.
Btw, I agree that the “sed ŝi” part in your sentence is not necessary, since it's not part of the English translation. But using the reflexive pronoun “si” is not correct here. “Ŝi ne strangas” or (better) “ŝi ne estas stranga” is a complete main clause with its own subject “ŝi”, and the reflexive pronoun can never be a subject, as it refers to its subject.