I'm not sure how your first suggestion would work, unless I don't understand what you mean. If the Property Test is accumulating the values, then you'd need some kind of secondary test to check them, otherwise it's a chicken and egg scenario, where you're asking the Property Test to accumulate values, but also test those accumulated values, which would be a different test altogether (a custom float value check, rather than a neighbor test).
For 'nth particle' tests you can just use filters to reduce which particles are affected by the operator (namely the filter which tests a random value between 0-100...set your condition to 'less than' and the number you choose to test will be equivalent to the percentage of particles that will be filtered through).
For 'nth particle' tests you can just use filters to reduce which particles are affected by the operator (namely the filter which tests a random value between 0-100...set your condition to 'less than' and the number you choose to test will be equivalent to the percentage of particles that will be filtered through).