The exclamation points and the way the sentence is formed. Otherwise it would be something like He look there is a cat in the kitchen.. Informal imperative not negative is the same as the third person singular form. Negative imperative (don't look) would be No mires. Imperatives are confusing.
I find myself confused--my understanding was that the imperative and subjunctive form for a regular -ar verb in this context is "mire" not "mira". I have the same question about the use of "gira" for the singular imperative in other sentences in the lesson. Am I misunderstanding something about how to form the imperative, or are these just exceptions?
"See" to me should be used as a question rather than an instruction.. As in, "See? There's a cat in the kitchen." Which would translate too "¿Ves? Hay un gato en la cocina." As a question, "see" sounds more natural in both English and Spanish. Hope this helps explain as to why it probably isn't accepted.
Yes. In my experience Mira is used very often for "Look!" (as in "look at what's over there") and as an expression before something that one is about to explain (as we do in English).
ves can be a verbal "tic" that a speaker uses -- kind of like how some English speakers might say "you know"