Would light being forced to slow down while passing through a medium

give the same effect as light slowing down over a vast distance and time? (hypothetically)

The now-overturned "tired light" idea of the 1930s is what I believe you're talking about. Since it is a purely hypothetical idea, you can say it is similar to that if you want, or you can say it is totally different. It is impossible to pin down one such idea and say "this is the way light 'should' slow down over a vast distance." It actually shouldn't, in any way at all, and there is no one tired light model that is any better than any others. So, take your pick.

It's like asking whether goblins dance in a manner similar to Lady Gaga. Since there is no evidence for them in any form, there is no reason to prefer either a "yes" or a "no" answer over the other.

The speed of light ALWAYS changes when passing through a medium, including air.

"...In optics the refractive index or index of refraction of a substance or medium is a measure of the speed of light in that medium. It is expressed as a ratio of the speed of light in vacuum relative to that in the considered medium.[1] [2] [3] This can be written mathematically as:

n = speed of light in a vacuum / speed of light in medium.
For example, the refractive index of water is 1.33, meaning that light travels 1.33 times as fast in vacuum as it does in water. (See typical values for different materials here.)

http://en.wikipedia.org/wiki/List_of_ref…

As light moves from a medium, such as air, water, or glass, into another it may change its propagation direction in proportion to the change in refractive index. This refraction is governed by Snell's law, and is illustrated in the figure to the right. Refractive index of materials varies with the wavelength of light. This is called dispersion and results in a slightly different refractive index for each color.[4]