1. Yes bclk will change the PCIE bus speeds, this usually is the limiting factor for bclk overclocks, can normally only get a few mhz out of the bclk.
2.CPU strap is something normally for RAM or even Xeon overclocking, On x99 anyway its hard to get ram over 2666mhz without setting a CPU strap to 125. Setting it to 125 keeps everything else at 100mhz including the PCIE bus. CPU strap only applies to x58 x79 and x99. Might be older boards that support it, I don't know.
3. Yes its safe, but you will have to change the multiplier to a lower value if you don't want the CPU to be at a higher clock. Im at 125 CPU strap with a 37 mulit, 4625mhz.
4. It depends, Normally you would want to max the multi and then squeeze a little bit more from the bclk. Or if you want more out of the ram you mess with the bclk, but you will be limited to at the most of 110mhz witch will OC the RAM somewhat.