I have two comments.
To start with your second observation, my only observation is that if you want to look at cross-national evidence and take all the good things that are higher on average in PR countries, you should take the negative things as well. It appears to me that you're more likely to have anti-immigrant, anti-legal immigration parties winning seats in PR systems in largely diverse countries than you are in majoritarian countries that have largely diverse populations.
To your first point, I think that this is a normative question, and the question is really this. You can imagine an array, after the next election, in which.... New Zealand hasn't completely fragmented as a system. The two principal parties, Labour and National, are still winning 40% of the vote or thereabouts in each election, but imagine an arrangement in which one of our traditionally larger principal parties wins 45% of the vote and forms a majority coalition with a party that wins 6% of the vote. They now hold the majority of the seats in the Parliament in that coalition.
Gamson's law would tell you that the power that would go to that other party would be proportional to their contribution to the seat share, so why is it that a party that got 5% of the vote ought to have 10% of the power within the cabinet? Why is that more desirable than a party getting 100% of the power on 40% of the vote, or 100% of the power on 45% of the vote? That's a normative question. I think it's one that the committee needs to explore, but the reality is that in PR systems, coalition governments are more common than in majoritarian systems. Coalition governments have some good that is attached to them, but they have some drawbacks, not least of them blurred accountability and behind-door compromises that occur after an election and between elections, not before elections.
These are normative trade-offs that have to be made, and the committee ought to consider them, but I've yet to hear a very convincing argument—and I'm open to being convinced—about why a party that has a very small percentage of the vote should receive such an outsized share of cabinet power when they're in a coalition, and why that's desirable—and perhaps it's not, right? I've yet to hear an argument as to why it is, and that's more likely to result in a PR system than in a majoritarian system.