おかし can refer to sweets in general, including baked goods. (Actually, I think "snack" is kind of an odd translation for it.) あめ is more like hard candy.

あまり is used with negative verbs/adjectives to mean "not really". It's the combination of あまり and おいしくない that makes it "not really delicious".

I had to actually look up 菓子 in the dictionary, because I've never heard it without the お. It's listed as a common word, so it seems you're right. I don't think it's commonly spoken, though.