Question on the disprove of CCA security
When the oracle gets $(r,s')$ this means that it gets $m'  =  0\mathbin\|1^{n−1}$ or $m' = 1\mathbin\|0^{n−1}$ because the oracle sent the adverary either the encryption of $m_0$ or $m_1$ that is $(r,s)$.
$0\mathbin\|0^{n−1}$ or $1\mathbin\|1^{n−1}$ is not the case since the bit flipping doesn't affect the $F$. The oracle will get $(r,s')$ then it will calculate $F(k,r)$, then the stream will be x-ored with $s'$. If we represent the bits by sub-indexes
$$F(k,r) \oplus s' = \left[F(k,r)_0 \oplus \color{red}{s'_0},F(k,r)_1 \oplus s'_1, \ldots, F(k,r)_{n-1} \oplus s'_{n-1}\right]$$ then we have
$$F(k,r) \oplus s' = \left[F(k,r)_0 \oplus \color{red}{\overline{s_0}},F(k,r)_1 \oplus s_1, \ldots, F(k,r)_{n-1} \oplus s_{n-1}\right]$$
As we can see, only one position has affected, the first position.