1) Often, 0V is switched because N-channel and NPN devices have tended to be cheaper. There's no safety issue, as there would be in high voltage.
2) RxC is an approximation for the time to trigger a 555, since the 555 2/3 trigger point is very close to voltage at which we define one RC time constant.
3) Transistors have a "current gain" or Hfe, which is the ratio of output/input current. If you give a transistor an input of 1 mA, and it has an Hfe of 200, then it will conduct up to 200 mA. Here, it gets complicated. To minimize power dissipation you want to "saturate" the transistor, and the saturated Hfe is generally a lot lower.