I don’t entirely trust single member districts due to gerrymandering, and even if the districts are drawn in good faith, I still anticipate that they will yield some distortion of public opinion in the legislature. Running simulations of PR vs Single Member Districts (for, say, a 5 member council) in the Wolf committee might be worth considering. Checking for the possibility of different kinds of strategic exploitation seems difficult, though, since there may even be strategies that we are unaware of. Perhaps machine learning could be used for this?

In section 6.1 of the Schulze paper I linked to, he argues that vote management is essentially large scale Hylland Free Riding. He gives the following STV election as an example:

10 voters a>b>c

35 voters a>c>b

25 voters b>c>a

30 voters c>b>a

In example 1, the winners are the candidates a and c. However, when the candidates a and b run a vote management strategy against candidate c and ask their supporters to vote preferably for candidate b then this example looks as follows:

10 voters b>a>c

35 voters a>c>b

25 voters b>c>a

30 voters c>b>a

Now, the winners are the candidates a and b. The fact that vote management is possible although only 3 candidates are running for 2 seats so that no elimination of candidates and, therefore, also no exhaustion of ballots occur demonstrates that none of these properties can be that property that is misused in a vote management strategy. This example demonstrates that also (1) the special rules to transfer surpluses or (2) the violation of monotonicity cannot be that property of STV methods that is misused in a vote management strategy.

That varies from election to election, so it would be difficult. Schulze said in his paper that his STV method minimized Hylland Free Riding so that it was only vulnerable in situations where this vulnerability was necessary to pass Droop Proportionality. Unfortunately, I do not know what such a criterion would look like for the PR criteria we use for score methods. For example, we could define PR as “if k Hare Quotas of voters score a set of at least k candidates max and all other candidates min, at least k of the max scored candidates should be elected.” Defining a maximal vulnerability criterion is not as simple as saying “suppose a candidate C would be elected regardless of the score a set of voters give to C, if no voters outside this set change the scores they give to C. Suppose that these voters change their scores for C in some way. Then the winner set must not change unless this would violate PR,” because this criterion is still not compatible with PR and Pareto. In the following Approval-PR election with 3 winners:

10 AD

10 BD

10 CD

No winner set outright violates PR, but Pareto requires that D be elected. If the first group of voters removes their approval of D, Pareto still requires that D be elected. PR requires that A be elected. Since the first group of voters were unnecessary for electing D, and electing A and either of the other candidates would not violate PR if D is approved, A must be elected in the original case by the “Maximal resistance” criterion. But B and C must also be elected for similar reasons. Thus the maximal resistance criterion is incompatible with PR and Pareto.

STV is probably more vulnerable to free riding than either your version of RRV or Warren Smith’s, Asking for different groups of your party to bullet vote for various assigned in a score-PR election is risky, since if you overestimate the “effective quota” or underestimate your support, the extra bullet votes will be completely wasted and you will overpay for your seat. If you underestimate the “effective quota” or overestimate your support, you may spread your voters too thin. In STV, if you do this, the votes can be salvaged: if you overestimate the quota or underestimate your support, you will only pay a fair price for your seat, and the rest of the vote transfers. If you underestimate the quota or overestimate your support, then some of your candidates will be eliminated and their votes will transfer to your other candidates.

Then again, since the effective number of votes needed to force election will be no larger than the Droop Quota, there will be a guaranteed window between the Hare Quota that a party would pay to elect candidates if they did no vote management, and the “effective quota”, parties may still deem it worth the risk, especially if their opponents are doing it, and they fear being “cheated” out of a seat.