An explicit computation in class field theory

Given that you want to know the structure of the Galois group and ramification, I think that you are best off working with the kernel of the norm map between connected components of idele class groups, as you yourself suggest.

These groups are very explicit: for $K := \mathbb Q(i)$, one obtains $\hat{\mathcal O}_K^{\times}/\{\pm 1,\pm i\}$, and for $\mathbb Q$ one obtains $\hat{\mathbb Z}.$ (Here $\hat{}$ denotes the profinite completion.) Apart from the diagonally embedded $\{\pm 1,\pm i\}$ quotient in the group for $K$, both groups factor as a product over primes, and the norm map is given component wise.

So the kernel of the norm map is equal to $$\bigl(\prod_p (\mathcal O_K\otimes_{\mathbb Z}\mathbb Z_p)^{\times, \text{Norm } = 1}\bigr)/ \{\pm 1,\pm i\}.$$

This should be explicit enough to answer any particular question you have.

In your particular case, $K^{ab}$ is completely understood, but your field is one of the very few for which such an explicit class field theory is known, so you got lucky.

I don't know your background, but to understand the answer you need to know something about the theory of complex multiplication. What I am going to say works for any imaginary quadratic field. The field $K^{ab}$ is generated by so-called Weber functions, usually just given by the $x$-coordinates of torsion points on any elliptic curve that has complex multiplication by the ring of integers of $K$. Actually, in your particular case, you are looking e.g. at the elliptic curve $y^2 = x^3+x$ and the maximal abelian extension is just generated by the torsion points (always true when $K$ has class number one).

You can read up on this in Silverman's "Advanced Topics in the Arithmetic of Elliptic Curves", Chapter II. Have a look particularly at example 5.8.1.

Use the theory of complex multiplication. $K^{ab}$ is the field generated by the torsion points of $y^2=x^3+x$.