How many successful CM's does it take to get a kill these days?

As low as 40 and as high as 130 atleast. New formulas means there are a lot more variables.

if 100% nw matched with tyr + dec war + max mil tech you could do it in 35

What's the minimum civs killed per CM?

under normal circumstances 25 prior to the kill, but if you have extremely low pop (like almost none) it is possible to see lower without a kill.

midpoint of set cm returns are higher than early set (1st week).

before mehul sold swirve council-server bugged and chems dealt low damage w/o countries being dead but bit overactive mods (patty, mr mickster) deleted those countries cause they thought those had been killed but they weren't so mods needed to reincarnate those countries.