Exclamation marks in written English (though texts etc somewhat different rules but then they can be mindless) best used with extreme care - ie: not too much. It's considered bad style/a sign of lack of confidence in how interesting what one's writing actually is. Kind of like going boom boom after a weak joke ( different rules for Basil Brush - google)
Grazie! E a proposito, Duolingo no accetta tuttavia la forma " E' ", solamente "È". :/
The 2nd person (sing/pl) are simply orders for informal you, tu/voi.
Both the 3rd person and 'entirely different form' are orders for the formal you, Lei. As far as I understand it, the formal imperative takes the verb in its subjunctive form, which may occasionally be the same as the regular third person (though I can't think of any examples) and the 'different form', such as 'faccia', 'venga', 'sappi' Don't quote me on the specifics, but basically it's due to the formal imperative taking a different form