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 writer is possibly referring to "hidden time." 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 writer of Psalm 90:2 to be saying that God has always existed in time and always will exist in time: God has no temporal beginning nor will he have a temporal end.
I concede that the attempt to plumb God's eternal 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. 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 unseemly implications flow therefrom. If God is timeless, God does not--probably cannot--change ontologically or cognitively. Furthermore, if God is timeless, he does not have genuine emotional states. Additionally, if God is atemporal, then Jehovah knows all things as present: all events whether past, present or future to us are all present to him. I have fleshed out these implications at other times.