You're right that after the light passes through the glass, it will resume it's speed of 3x10^8 m/s.
And your second question has to do with relativity. I can't remember the exact calculations to find the relativistic speed of light in reference to 2 objects, but that's what 3 months of summer will do to you. But 4 months ago if you asked this, I woulda known the exact answers and be ashamed the mighty Ash didn't know.
I read something on CNN a few weeks ago how scientists believe that the speed of light may have changed over time. I didn't read too much of it in depth as Physics in realistic applications is never an exact science. But these findings would refute Einstein's photoelectric effect theories.