It's probably an average. Industrials 10% S&P 20% Nasdaq 30% if memory serves

from the very limited reading I've seen into the subject the idea is it would be detectable in the CMB - something along the lines is it would affect the distributions of bands or something like that.

If you do some digging for papers on it there's are suggestions on how to test for it / physical consequences

Admittedly I wouldn't be the best person to ask as I haven't studied science in nearly a decade, but my uneducated educated guess would be a change in the value of c is balanced out by other factors, probably something to do with dark energy.

In a sense the changing rate of expansion of the universe due to dark energy is what is causing the change in c. But that's just 100% made up of the top of the bonce. Hopefully someone with a more advanced and recent education can give you a good answer.

Key point is things don't change just because. There's always a reason fundamentally

I don't think it's out there at all and it could be a fascinating avenue of research haha. I wasn't trying to be dismissive - I just know there's been a few papers put out speculating on a changing value of speed of causality, just I don't think there's been enough evidence or testability to really "mean" anything scientifically

In short, yes.

In not short, there are some more fringe theories coming out of quantum mechanics that posit that the speed of light (or more specifically the speed of causality) has changed slightly over time. However I've never seen anything serious come out of such papers.