How are the Ignition Cut and Fuel Cut severities implemented, does it just take this percentage of fuel away on every cylinder?

No, the cut omits fuel injection pulses to an average number of cylinders, that is to say that if you have a 33% cut severity (ie one in three) on a n*3 pot engine then it won’t always be the same cylinders that aren’t fuelled, but over a number of revolutions you’ll achieve the given average.