Answer to Question 1
From the Sun inward, we measure about 100 billion solar masses from the speeds and distances of objects around us in the disk. But when the huge quantities of dark matter out beyond us in the halo is considered from the Galaxy's rotation curve, the dark matter adds up to enough to push the total to 600 billion Suns.
Answer to Question 2
The density wave moves slower than the stars do, based on the Doppler shifts of the stars compared to the gas and dust that forms the spiral arms.