eine is the only number that inflects, in general. (Though some of the next few numbers have distinct genitive forms, e.g. er ist der Vater zweier Söhne "he is the father of two sons".)
Das sind die letzten blauen Orangen is correct -- both adjectives take weak inflection after the definite article die there.
If you listen carefully, you can hear the "s" (like in "snake") of "das" before the "s" (like in "cars") of "sind".
You're right: "Da sind die letzten zwei Orangen" is also a valid sentence, meaning "The last two oranges are over there." However, if this were the case, I think the speaker would most likely place greater emphasis on the word "da".