Right, and more specifically, even though it is in the imperative mood, the sentence by itself is not necessarily an order; it could also, for example, be the answer to the choice question "Should I drive the car, or do you want to drive the car?" In that case, the answer is still "You drive the car" in both English and Turkish, but the meaning is different, even though both uses are grammatically identical in writing. That is to say, you'd need either contextual or tonal clues to differentiate the cases.
In any case, 'sen' is a hidden subject in the sentence "Arabayı kullan" so it is assumed anyhow, mostly because of the way the verb is conjugated. In fact, you would add 'sen' in parentheses when analyzing the sentence and mark it as a hidden subject: (sen) arabayı kullan. There are some cases where not including the subject can disrupt the flow of conversation or cause confusion, though.
Come to think of it, if you assume this sentence is the answer to the choice question proposed above, 'sen' would now be REQUIRED, as it is a direct answer to "you or me?" and to complete the question-answer pair, you need the actual answer. So the answer to the question "Arabayı ben mi kullanayım, yoksa sen mi kullanırsın?" needs to have either 'sen' ("Arabayı sen kullan / You drive the car.") or 'ben' ("Arabayı ben kullanırım / I will drive the car.") as a subject. That is, you cannot use a hidden subject to answer a question asking you about the subject, so 'sen' would be required.
As you said, using an explicit subject also can be used to differentiate exactly who you want driving the car out of a group of people. In that case, it would not be uncommon to hear it coupled with a name, such as "So and so, you drive the car." I guess in that sense, you'd expect the subject to be explicit, but no meaning is lost if you do not include the subject, i.e. "So and so, drive the car." I cannot think of a situation in which an actual order would lose its meaning because the subject is hidden, but I'm sure there are cases for which it does.
So while it is generally true that the subject is optional, there are many cases where it is either mandatory or expected. When in doubt, I'd say include it. The worst it will do is sound awkward, which is much better than not being understood at all.
Nişanyan's etymological dictionary compares it to a Sanskrit word and says the word was probably loaned into Turkish from some Iranian language, but that the sound changes involved are not clear:
OTü araba at arabası ≈ Saka rraha a.a. ≈ Ave raθa- a.a. ≈ Sans rátha- रथ iki tekerlekli tören arabası
Not: At arabası ilk kez MÖ 2000 dolayında Ural Dağlarının güneyinde Proto-Indo-Aryan diline mensup Andronovo kültürü tarafından imal edildi. Türkçe sözcüğün bu dilin devamı olan bir İrani dilden alıntı olduğu muhakkaktır. Ancak ses evriminin mahiyeti açık değildir. • Kaşgarî sözlüğünde yer verilmemiş olması, 11. yy Türkçesinde henüz "yabancı sözcük" olarak algılandığını gösterir. • Ar ˁarrāda(t) عرّادة ve Lat raeda "araba, özellikle iki tekerli tören arabası" İrani dillerden alıntıdır.
Like many Turkish words (or so it seems to me), kullanmak has a range of meanings.
The core meaning is "use" but when the object is araba, a translation of "drive" makes more sense.
It is - it's the imperative form, which is formed by removing the infinitive ending -mAk and adding nothing.
So you could say that the imperative involves a zero morpheme -- a bit like how in English, you can take an infinitive such as play and add -s to get he plays or -ed to get we played or -ing to get they were playing... or you can add nothing to get I play. That's also a conjugation, just one where you don't add anything.