Dark sesame seeds are more common in Indian cooking. I personally think that they have a more intense taste. They are also smaller, for what little difference that makes. I wouldn't rush to use them to make tahini, because the color would be surprising and I suspect that the flavor would be a bit bitter.
Fennel and caraway are relatives, but not the same plant. Fennel seeds have a flavor dominated by anise/licorice, where caraway is quite different, being dominated by other flavors. They also have subtle differences in appearance.
Cumin, anise, and dill are other look-alike seeds with very different flavors. Anise and fennel have very similar flavors, and can be substituted for each other. Caraway and dill are likewise similar in flavor. Cumin has a flavor different from any of the above. I am including pictures of fennel, caraway, anise, cumin, and dill seeds so you can see how similar they are, but also how there are subtle differences in shape.
Fennel Seed:
Caraway Seeds:
Anise Seed: It is hard to tell from the picture, but anise seeds are much smaller than fennel.
Cumin Seeds: note, these vary considerably in color
Dill Seeds: (note how much flatter theses are)
Best Answer
Yes, they're all ready to eat.
Washing them is just going to get them wet, and it'll be difficult to impossible (especially for the flax) to get them dry enough to grind to a powder.
People do sometimes soak flax seeds, or mix ground flax seeds with water, in order to get something to use as a vegan egg substitute, but I don't think that's what you're going for.