(in particular, I think that it is definitely false in the totally imaginary case that the commutator subgroup of $\text{SL}_n(\mathcal{O},\alpha)$ is $\text{SL}_n(\mathcal{O},\alpha^2)$).

@Ian Agol : Bass-Milnor-Serre actually say which finite cyclic group it is, though the answer is a little complicated, and they give an explicit description of the isomorphism between $Q_n(\mathcal{O},\alpha)$ in terms of power residue symbols. But I can't extract explicit matrices from their description. I also don't know what the abelianization of $G$ is in this case; for the rings of integers where I know how to calculate the abelianization of $G$, that calculation depends on the CSP holding (so it doesn't work in the totally imaginary case).