I am not expert enough in astronomy to fully answer your questions. I was a solar astronomer for awhile. But triangulation and the constancy of the speed of light were not issues in that work.
First regarding triangulation, and remember that you asked me what I thought. You imply that the measurements are within the angular diffraction spread of the telescopes involved. In essence you are asking if the astronomers who are making such measurements are fools. I do not think that could possibly be true. Yet they have measured distances beyond 6,000 light-years.
So then you say that perhaps the speed of light was not constant during that amount of time or over that distance. Well the speed of light does not matter in a triangulation measurement.
But the point is that there is a tremendous number of scientific measurements from astronomy to geology that indicate that the earth, the solar system, the Milky Way and all other galaxies that we can detect are older than 6,000 years. So the belief of Orthadox jews that it is only 6,000 to 7,000 years old is just plain wrong. That makes the bible wrong and therefore not the word of God, at least not a perfect god.
So I can appreciate your concern with such a profound division between religion and science. But actually it is only a small part of religion that continues to believe in a 6,000 old universe. Neither the conservative jews nor the reform jews have that belief as official. And the number of orthodox jews that I have worked with, all scientists by the way, do not believe it either. They are just following a tradition. For them it is their actions that count, not their beliefs.
In Christianity neither the Catholic church or the Orthodox churches nor the protestant religion I was brought up in believed in a 6,000 year old universe. So compared to the totality of religions and religious people in the world the number that still believe in a 6,000 year old universe is miniscule.