Your sentence works. However, I think they are trying to make us understand that word order in Finnish changes the meaning similarly to marking definiteness in English. So the meaning of your translation would be closer to whichever of the following two sentences is grammatically correct: "Lähellä sientä hiiri on piilossa" or "Piilossa lähellä sientä on hiiri". (I would love if a Finn could confirm this for me)
In some situations, yes. Like 'Tuolla on mehua': "Some juice is over there".
But I don't think 'Sientä on herkullinen' works. It's much more common for a singular subject that precedes the verb to be nominative. And if it's partitive, I'm doubtful it can take a nominative adjective. There's just a little mentioned on this topic here.
However, I think 'Sieni on herkullinen': "The mushroom is delicious" would be a possibility.