They study distant galaxies because they want to know whats out in other galaxies and how many planets it has
So they can undersatnd what is producing the light, and if the galaxies are hospitable to other forms of life :)
That is impossible to answer because scientist don't know ALL the galaxies.
Moving away from us
The light from distant galaxies is redshifted. The only reasonable explanation for that is that the galaxies are moving away from us.
The apparent redshift is caused by the Doppler Effect; the frequency of waves is increased by an approaching source and decreased by a leaving source. Since the fgrequencies we see from stars are slightly lower than they should be, we know that the stars and other objects are moving away. The Doppler Effect causes changes of light emanating from distant stars and galaxies.
The "redshift" refers to the fact that the light is less energetic than when it was emitted; it shows that the galaxies move away from us. Should a galaxy move towards us - which is possible only for galaxies that are relatively close to us - then there would be a blueshift.
visible light
The bending of light due to great masses is called Gravitational MicroLensing.
If you look at a distant galaxy, the light from the galaxy has travelled for perhaps a hundred million years, a billion years, or up to an age close to the age of the Universe (13 billion years or so), depending on the galaxy's distance. Thus, the light you see shows you how the Universe was billions of years ago.
Yes. Lensing magnifies the image of galaxies behind distant galaxy clusters but also greatly distorts the image.
It doesn't. Light comes to us from the Sun, and from distant stars and galaxies, through what is basically empty space.
No. We already know a good deal about the speed of light. Scientists study other galxies to learn how galaxies work.
The galaxies beyond our own are millions to billions of light years away, meaning the light takes millions to billions of years to get here.