What exactly were the "Keruvim"?
Rashi, Ramban (on Pasuk 21, citing Chagigah, 13b and Rashbam (citing Succah, 5b): Cherubs with the face of a child. 1
Rashbam: A large species of bird.
As in Yechezkel, 28:14. As a matter of fact, the word "Keruvim" really reads 'ke'Ravya', like children (Ramban). The Ramban, who hints according to Kabalah why the Keruvim faced each other and why they were 'beaten'. See also R. Bachye, who gives a detailed explanation of the Keruvim, and the Seforno on Pasuk 20.
Why does the Torah write "Shenayim" and not "Sh'nei"?
R. Bachye: Whenever the Totah writes "Sh'nei" it means that they are exactly similar, 1 which is the case here in most regards. Only they differed inasmuch as one had the face of a male, the other, of a female. 2 And it is to indicate that they were to be formed as a couple (and not as two individuals) that the Torah writes "Shenayim".
What is the definition of "Mikshah"?