Yes, using %victim and %killer does work. Try this:
Player Kill Player:
Set Variable (%victim killstreak = 0)
Set Variable (%killer killstreak += 1)
This will reset the victim's killstreak and increase the killer's.
I don't think there is a Vanilla way to check if a player is looking at a specific client-side block, so this might not be possible. Because the block is client-side, that means the server (probably) cant see them. They can only be placed by the server, not checked.
You can use " Set Variable: Set to Random Number " and set a variable to 1 - 100 or something higher if you want larger selection range.
Then, place an " If Variable: Value Is Within Range " and put the variable you made with the random number along with two numbers.
If you want to have a 50%...
Yes.
I was thinking about it for a little bit, and a GUI with each event that you have used in it.
Clicking on one in the GUI would teleport you to the adjacent event block.