It turns out that Einstein's assumptions are nearer the truth than Newton's. Contrary to simple expectation, it is not easy to detect a black hole directly, you need to look at its effects on nearby material. If there is gas nearby, it will heat as it falls into the hole and emit X-rays. Since the region is very small and chaotic, the emission will vary rapidly. This is the most effective way to find black holes.
Globular clusters are the old metal poor clusters that are found throughout the halo of the galaxy.
They were used by Shapley to pinpoint the center of the galaxy, because he assumed they were in a spherical distribution centered on the center of the galaxy. It is certainly not any of the other values which are way off. Remember, dust in the disk of our galaxy prevents us from seeing very far in the optical or ultraviolet spectral regions. Of course, infra red or radio telescopes CAN see to the center because the dust is less opaque to these longer electromagnetic waves. Remember, it is simple unionized single hydrogen atoms that produce 21 cm wavelength radio emission, when the electron flips its spin direction.