I didn’t even bother reading the rest of this thread, beyond the original question. I’ll try and answer, as one who has immersed relatively deep in Chabad teachings over the course of a number of years.
I will just say one disclaimer: there have been shifts in focus over the years. Every three years in the history of Chabad Chassidus, the target/goal attained a new depth. For instance, the three generations before the last Rebbe ZY”A, the emphasis was on Kabbalas Ol. Then the last generation was about a deeper level of Mesirus Nefesh.
You ask for only a couple of sentences, but it is VERY difficult to mention in so few words concepts which heavily rely on jargon — in other words, predefined, and previously explained topics and concepts which build up to a more essential theme. But I guess I’ll have a go at it, though I am not at all an official voice in Chabad teachings today (some notable examples of official voices, so to speak, might be Manis Friedman, YY Jacobson, YY Gurevitch, Yitzchak Goldberg, and of COURSE R’ Yoel Kahn and R’ Gopin, the latter two at the top of the list). People study these Chassidim’s teachings (obviously there are many more, but these are some who I am acquainted with personally) for many years before really grasping the quintessential objective of Chabad, and on the contrary there are many who can learn for years without really grasping anything but what their own imagination compels…
So here goes! (I’ll purposely try and avoid the jargon I mentioned above)
I would succinctly summarize the main goal of Chabad Lubavitch, that through the learning and deep understanding (and connection to) the study of Chassidus, the entire Earth should have an awareness of a superior, divine aspiration and ambition originating from a Superior Being (G-d), to be a relevant and inseparable part of physical life within it. This awareness should be so manifest in our conscience that it will affect us to practical actions to enact and make practical this ambition of G-d, in such a manner that it will become more important to us than any part of our own selves. (Or, to end in another fashion: it will become so important to us, that it will reach our inner core, at such a level that the only thing that really matters is to make this happen).
I would succinctly summarize the philosophy of Chabad, that at the very basic core of all existence is a simple unity which connects us to our Creator, Who in turn desires in us an intimate relationship, and of us to reveal this unity in every single aspect of our lives. This in turn inundates every aspect of every individual’s life with deep meaning, all as a result of indulging fully in the teachings that Chabad’s Chassidus has to offer, in a manner of TRUE understanding and connection