[aimhigh]

Thus scientists measure the amount of this red shift to determine the receding speed and thus the distance, in light-years.

Of course this assumes the red shift is totally unaffected by whatever forces, known or unknown, exist in the path of the billions of light years traveled.

[Hebrews 11:6]

You're quite correct about that assumption, but you're not the only one ever to point that out. Astronomers have invented various means of testing it, with very satisfactory results. Full details are widely available.