= Solution
Let $\mathbf n=(\cos\theta,\sin\theta)=(c,s)$ with $\theta=\theta(x)$. Then
$$
(\nabla\mathbin\cdot\mathbf n)\mathbf n
+(\mathbf n\mathbin\cdot\nabla)\mathbf n
=\theta'(-2sc,c^2-s^2).
$$
The squared norm is $(\theta')^2$ because
$$
4s^2c^2+(c^2-s^2)^2=1.
$$
Thus the <one-elastic-constant nematic free energy> becomes
$$
\mathbb F_{\rm elastic}
=2K\lambda_0^2(\theta')^2
=\frac{\widetilde K}{2}(\theta')^2,
$$
with
$$
\boxed{\widetilde K=4K\lambda_0^2=-\frac{Ka}{b}.}
$$
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