As you are well aware, the NWT translates OLAM in Psalm 90:2 and elsewhere as "time indefinite" and I think this is enlightening since the etymology for the Hebrew term suggests that the Psalmist is possibly referring to "hidden time, long" (Gesenius). It seems to me that Psalm 90:2 is not just enunciating God's "eternal" past but also the fact that God's existence is endless or boundless. That is, I take the Psalmist to be saying that God has always existed (in time) and always will exist (in time): God has no beginning nor will he have an end.
I concede that the attempt to plumb (explore fully) God's eternal/everlasting psyche is probably futile. Of course, Trinitarians will say that God was never alone in the first place, although Tertullian does venture a theological opinion regarding God's solitary existence and what God was possibly doing before he generated his own Word, thereby making that impersonal Word the Son of God (a personal entity). Tertullian's account can be found in Adversus Praxean 5-7; it is undoubtedly based partly on Genesis 1 and Proverbs 8:22ff. His suggestion is that God conducted discourse within himself when alone much like a man or woman thinks or deliberates when by him/herself.
The mode in which God knew prior to creation or knows now is a tough question. But I'll just say that I'm very much opposed to the absolute divine timeless idea when it comes to Jehovah since a number of implications flow therefrom. If God is timeless, God does not, nor can God change ontologically or cognitively. Furthermore, if God is timeless, he does not have genuine emotional states. Additionally, if God is timeless, then God knows all things as present: all events whether past, present or future to us are all present to him. I can flesh out these implications at another time.