Exercise 6.6 Feedback wrapped around an NPN - limiting factors

a)

An answer:

b)

c)

An answer:
Then the specified equation is not valid anymore: the term that includes the $v_{\mathit{CE}}$-dependency into account must be included. That shows that $i_C$ is (almost) zero for $v_{\mathit{CE}}$ that is (almost) zero. The opamp tries to compensate this by letting $v_X$ as high as possible (usually limited by the $i_B$ that grows exponentially with $v_X$).