From what I understand, the Orthodox declared all within the German Reform movement as karet, cut off, non-Jews, and they don't recognize any Reform Jew as Jewish to this day. However, some of these Reform swung to the center to become Conservatives, and so descend from the cut-off Reform. Therefore, Conservatives aren't considered Jews either. Is that correct?
This is totally incorrect. Even if a Jew sins sufficiently to merit being punished with karet, he is still a Jew. An example would be those who succumbed to the Spanish in 1492 and converted to christianity but did teshuvah (repented) and managed to escape to Amsterdam.
While it is true that Reform (as a movement) is not Judaism in the eyes of Orthodoxy, those whose mothers are Jewish (according to halacha) are still Jewish and subject to halacha. Of course, most of them nowadays are in the status of "tinok shenishbah" (captive infants - never having learned proper behavior) but they are Jewish.
Another example is found at Is a Jew Who Converts Still Jewish? which explains the matter. The best way to explain it is
Even if a non-Jew would marry a Jew with a chupah and a rabbi presiding with all the procedures “by the book,” the marriage does not have the validity of a marriage sanctified in accordance with Jewish law. Saying that “they are considered married” is the best Talmudic language available for “Yes, he is still Jewish.”
Based on the above statement of the Talmud, the Jewish Code of Law4 rules that a marriage between a Jewish man and a Jewish woman who “convert out” is completely valid. Therefore, their children are considered Jewish and could also marry other Jews.