I don't think your citation supported your original claim that, "Actually everything back to about 150 million to a billion years
after the the event is pretty much theoretical."
The cosmic microwave background radiation was produced before the universe was a half million years old. From http://en.wikipedia.org/wiki/Cosmic_microwave... :
"As the universe expanded, adiabatic cooling caused the plasma to lose energy until it became favorable for electrons to combine with protons, forming hydrogen atoms. This recombination event happened when the temperature was around 3000 K or when the universe was approximately 379,000 years old. At this point, the photons no longer interacted with the now electrically neutral atoms and began to travel freely through space, resulting in the decoupling of matter and radiation."
And Big Bang nucleosynthesis was complete by about T = 1000 seconds. From http://en.wikipedia.org/wiki/Big_Bang_nucleos... ;
"Big Bang nucleosynthesis ... began at temperatures of around 10 MeV and ended at temperatures below 100 keV. The corresponding time interval was from a few tenths of a second to up to 103 seconds."
Have you read The First Three Minutes by Weinberg?
Yes it is theoretical, but supported by evidence. That can't change the theoretical basis. We can directly observe the exact same things that happened after 150 million years to billion years after , happening over again.
Any look into the unknown by experimentation, such as the Big Bang is a branch of theoretical cosmology. If we could see back farther
we would have a physical model but are limited in that respect.
At this point we can say anything beyond a certain limit is theoretical. This image shows you the limit. though it maybe extended a bit farther in a few years. At any rate there is a gap between what is known for sure and what is under study, what is under study is of theoretical nature.