Do you know what classes of voting methods this impossibility proof applies to?
1. Does it apply to approval ballot voting methods for which candidate X can when more then 1 seat? Allowing X to win multiple seats so reduces the need to free ride, and as a result, it significantly reduces the need for a voter to betray their favorite candidate when that voter's favorite candidate is the candidate to which is guaranteed to win at least 1 seat.
2. What quota does the proportionality requirement use? Does this proof only apply to voting methods in which a candidate with 1/(total seats + 1) % of the voters bullet voting for them is required to win a seat? Does it also apply to voting methods in which a candidate is only required to win a seat if 1/(total seats) % of the voters bullet vote for them?