On that last point on representation, you also have think about minor parties. The old idea of parliamentary representation is that you go through your MP, and irrespective of party, you are representing the constituency you're a member of. It does mean that small parties are excluded. If you have a mixed member system, smaller parties are more likely to be there, so you don't necessarily need to go through your MP. You can go through a different channel to get representation.
In terms of rank preferences, essentially that's another choice. It's a more majoritarian system if one goes toward a preferential vote, and that has certain consequences for party representation, but I don't think you should think of it as a sequential step. It's basically a choice that you need to make, and you don't want to say, “Let's have that, and then further down the road...”, because you don't want instability. Every electoral system takes time to work out what the consequences are, particularly for voters in knowing how to act within that choice in terms of the ballot and in terms of districts. You don't want to have two choices.