Yup, "magas" has been "tall" everywhere in the lessons, and suddenly became "high". I guess the Rocky Mountains are "high" though.
No, they are also tall. Tall/short are descriptive adjectives. High/low are comparatives.
Stores are not tall, buildings are tall. Stores may be in tall buildings but they themselves are not tall. Or do Hungarians use "store" and "shop" as a kind of building?
To talk of a "magas üzlet" sounds a little weird (I'd rather just say üzlet without a descriptor), but there's factually nothing wrong with it. Üzlet can also refer to the store building itself.
One would hope because shops are not tall. Buildings are tall. Shops are not buildings, they are an abstract concept meaning "place where one trades money for goods." Well, that's one meaning. "Shop" can also mean "the place that fixes cars," or "the space in my garage where I do <stuff>."