Question #1- If a person davening without a minyan recites the first and last of these kedushos, why not the second?
It's actually a machlokes. Shulchan Aruch (OC 59:3) writes:
יש אומרים שהקדושה שביוצר יחיד אומרה לפי שאינה אלא סיפור דברים ויש אומרים שיחיד מדלגה ואינה נאמרת אלא בציבור ויש לחוש לדבריהם וליזהר שיחיד יאמרנה בניגון וטעמים כקורא בתורה. הגה: וכבר פשט המנהג כסברא ראשונה ויחיד אומר אותה
There are those who say that the kedusha in yotzer is said by an individual, since it is only a recitation of a [Biblical] narrative, and some say that an individual skips it and it is not said except in the public. One should be careful to follow their words and be careful to say it withwith the tune of the ta'amim as a reader of the Torah.
Note (Rema): The custom has spread as the first opinion, and an individual should say it.
Thus, according to those who say that an individual does say the first one (during the beracha of yotzer or), the reason is because they're merely repeating a narrative, not actually saying the kedusha. The same reasoning applies to the kedusha of Uvo L'tziyon.
Question #2- If the second kedusha "belongs" in the sequence of berachos of the amida, why remove it if there is no minyan?
I don't know of any sources that say that it "belongs" in the sequence of berachos of the Amida, in fact it appears to be completely unrelated to the berachos of the Amida.