4.2GHz is a small overclock. A Cooler Master Hyper 212 EVO can handle that easily. Even up to 4.5GHz. For higher, there are premium ones like Noctua NH-D14 or Phanteks PH-TC14PE which cools just as well as a H100.
As high as you can go or as high as you are comfortable with. I don't know a direct answer to what overclock would get you your money's worth. The chip itself is worth its price already IMO. So any OC is just extra.
Start experimenting and see how high you can go. Go in baby steps. Don't jump to 4.7GHz just to start. Also, just because someone managed to get their CPU to 4.7GHz OC, it doesn't mean that your chip can. All chips are not created equal unfortunately. Yours may be better; it may be worse; or it may be somewhat equal.
Wrong, LGA 1150, 55 and 56 use the same mounting hardware.
The Phanteks cools just as well as the Noctua-but with color options.
The H100i is better for 20 dollars more, and NZXT Kraken x60 is even better for around the same price of the H100i.
212 Evo is best price/performance, doesn't mean the best cooler.