Hey! Yes,they are interchangable.

turks have ilkbahar which means first and sonbahar is fall.sonbahar is the second bahar or last bahar. Son+bahar, is son here means second or last?

Doesn't "ne" mean "how"...therefore it would mean "How nice is spring?"

In this sentence, it also works.

Duo did not accept it .... although it means (so, too.....)

Yes it is optional. Generally we use it in general statements.

ps. The correct sentence would be "İlkbahar çok güzeldir." (not -dır) due to vowel harmony.