Is "quả" a Northern way of saying "trái"? B/c my family is from the south, and I have heard only "trái"
Yes, but quả is also more poetic, and is used more in literature. Quả táo conjures an image of a perfect apple for me (personally), shiny and round, something to cherish, while trái táo is just an apple, nothing special :-)
Depends, in fact you would most often hear trái tim as opposed to quả tim in poetry. Quả is just a Sino-Vietnamese borrowing while Vietnamese poetry often uses native terms, hence, trái.