In practice, states decide how to vote in both congressional and presidential elections (Maine for example, uses ranked choice for both). It is true that getting rid of the electoral college requires a federal amendment, but the electoral college isn't actually that bad; the big problems can be solved within its framework
Tl;dr: We should argue before the election, and build a consensus when it's time to actually get stuff done
I don't think reducing (tribal) polarization is at odds with agonism. There's plenty of room for healthy debate about what direction we should go in, but when in comes to actually deciding who has power, we want to track the median of what people think, not sway back and forth between two extremes (which is a compromise in and of itself)
I think it's worth being aware of lock-in effects; while I agree that it's unproductive to put down alternative approaches, it's also important to do what we can to avoid locking in a suboptimal choice- if Ranked Choice gets locked in over Approval Voting, and if Approval is actually better, it may be very hard to change it in the future, leading to much long-term disutility, perhaps even more than if the shift away from FPTP takes a little longer in the process of getting it right.
But I do agree that bickering and putting alternatives down probably isn't the best way to mitigate lock-in
From a total view, I think this does outline a potentially compelling case against the climate change argument, but I don't think it's compelling from an average point of view. Even from an in-between perspective (which I think roughly represents my feelings) which evaluates overall welfare as the product of average quality of life times the square root of population, it seems that marginal hits to the climate may outweigh marginal gains in quality of life.
Even from a totalist POV, it's important to consider lives with negative value. It matters where the line is drawn- some people may feel all lives have positive valence, others may set a high standard, and say that a life with positive valence must have very little suffering, even an absence of suffering that may be fairly unheard of in modern life. By having children, you might increase the value that your life contributes to overall welfare, and contribute the lives of your children, but have an impact on many people which changes their lives from making a positive contribution to a negative contribution, or greatly increases the suffering of a person who already has a negative-valence life.
This certainly isn't to say that I think you're wrong - I think the structure of your argument may be usable to make a compelling case which addresses my concerns, and in general I do feel (though am not certain) that on net, the average person will contribute more to other's well-being than they take away.
In short, I think the effectiveness of altruism is limited in a non-demopistic society.
My perspective is that markets are a key factor in enabling things to happen. EAs are focused on promoting the social good, but current markets only promote individual benefits, not public benefits. We can either work upstream against the structure of incentives that currently exists, or we can implement systems (such as Quadratic Funding) that align incentives with the public good, thereby making our work easier and motivating more people to work on projects that the EA community values.
But we can decide what goes inside the machine, whereas with people we can only control outside circumstances. It seems to me that such a mechanism would be highly likely to be an internal mechanism, so wouldn't be applicable to people