Why doesn't the photon have mass? The Higgs mechanism and pre-electroweak epoch

1) When electroweak separation occurred, 'why' wasn't the photon 'given' mass like the W and Z bosons? i.e why don't photos interact with the higg's field?

2a) How well is the higg's mechanism understood at the moment?

2b) Since the discovery of the higg's boson, is the process behind particles gaining mass something which is just accepted to have occured until we can explain it?

3) A slight abstract extension from this - before the electroweak epoch (i.e. in my understanding, the time before interactions with the higg's field occured to yield particles' mass) was the universe made up of a sea of massless particles? or at this very early stage, was the universe made up of pure energy rather than matter? (i know strictly speaking they are one and the same, but for the purpose of asking this question I am making a definition between the two.)

Thanks.

1. The $SU(2) \times U(1)$ electroweak gauge theory has 4 symmetry generators, of which the vacuum breaks only three -- corresponding to $W^+$, $W^-$ and $Z$. The vacuum is symmetric under the generator of electromagnetism, hence the photon cannot interact with the vacuum. Think of the vacuum as a medium -- if it was not symmetric under EM, then it could absorb a photon and transform into another state in the multiplet.