First of all, I don't think we're sure that even every large (Milky Way-sized) galaxy has a central supermassive black hole, let alone every galaxy. There are a few significant galaxies where a central supermassive black hole has not been observed.
Nonetheless, spiral galaxies like the Milky Way have a "flat rotation curve" which means that the velocity of the stars is constant as a function of distance. So, stars at the outer edges of galaxies aren't moving faster than stars closer to the center, it's more likely they have a similar speed (so the orbital period of stars further out is still longer than stars closer to the center).
I don't think this time dilation explanation is feasible. Such a magnitude of time dilation would require a galaxy to have orders of magnitude more mass than the notion of dark matter requires. And in general, invoking special relativity for a theory is not "a bit simpler" than using Newtonian mechanics (Occam's razor and all that stuff, you know). Simply assuming the mass distribution of the galaxy is not proportional to the visible, baryonic matter (stars, gas, and dust) is a perfect explanation, which is why we believe dark matter is an abundant part of a galaxy's mass, in addition to the fact that we have observational evidence of its existence.
Also, in regards to 2, I don't think time dilation would affect the time it takes for light to reach us, since photons don't "experience" time and they always travel at the same speed, but I'm not sure. Maybe there would be funky stretching of spacetime. I haven't learned too much about relativity and stuff.