Also, it seems scharf can mean pungent, at least according to Duolingo.
A pungent smell is a strong smell. It's a word that applies to tastes and smells. "Pungent" isnt necessarily a bad or good smell, but it's usually something that is strong enought to be unpleasant because of thr strength. So onions sitting in a hit car for a while would be pungent, but freshly fried onions that make the kitchen smell like food probably aren't going to be described as pungent.
No, it's exactly how a strong- smelling or tasting onion is described. "These onions are really sharp," one says, weeping over the cutting board. Or "really strong." But "hot" or "spicy" isn't right. "A sharp, oniony flavor." "Spicy" is more likely to be mustard or peppers.
Yes and no. Würzig means spicy as in heavily seasoned (with any spice), while stechend can mean pungent (or strong, biting, intense, etc. referring to taste or smell), but spicy as in hot is scharf which also means pungent (or sharp as in a blade, or many other meanings).
No, you cannot translate scharf as smelly or stinky. First of all, it is talking about taste not smell. Second, smelly/stinky both mean a clearly unpleasant smell. Scharf is not negative. It just means spicy (or pungent, biting, etc.). Depending on your taste preferences, you might dislike it, but it is not inherently bad.
One thing that would help you read & pronounce German, and also remember the spelling correctly, is to notice that when there is a combination of i and e, you pronounce the second one as it sounds in English. Some examples: the number two, zwei, is pronounced "zwhy". The word for "she," "they," and "you" (formal) is sie /Sie, pronounced "see". Start noticing that, and you will see it's true for all words and it will help you spell the word correctly when you hear it or you are remembering how it is pronounced.