I found this question pretty interesting, as I am pretty skeptical of most cosmological theory presently promulgated. There are a lot of assumptions, and little evidence to back much of it up.

So, I looked about.

I found this site, which discusses how plasma can interact with light, such that 'laser-induced structures in plasma that act as high-power mirrors.' are being used in efforts to fuse atoms.

There's more, but your thesis that plasma might impact light is definitely proved by the fact that plasmas both are used to reflect lasers and aim them, and are manipulated by lasers to make them act as mirrors.

It is almost certain the heliopause impacts the light passing through it.