But - generally speaking - if you’ve got a delegate you support in the parliament you’re much better off than if you’re casting a protest vote for an individual or group who will never hold a seat.

And you're better off still if you contact your representative about issues that matter to you in a community representation system.

Also people like Bernie Sanders or AOC simply wouldn't have any prominence in prop rep system. Bernie is an independent that's popular in his state. AOC is in congress because she won a primary in a safe blue district. They aren't required to vote on party lines, so the Democratic party has to compromise with them. In a prop rep system they'd either have to fall in line with the party leadership or form their own party and be irrelevant.

Third Parties are only relevant because of their potential to spoil an election. In a multiparty system they no longer have that capability. The only power they could potentially have is in the backroom deals to form a coalition with a larger party if the larger party doesn't have the majority of the votes. And once again, this kind of thing swings both ways. A center right party may need to form a coalition with far right extremists in order to take power, as we've seen happen in Israel.

Not if they’re doing the one-term Senate gambit, like Kristen Sinema. Six years cultivating favors with corporate interests, and then resign before you party can primary you out so you can take a job as a lobbyist.

Next election, Kristen Sinema will be gone. This is an indication of the system working, but you're characterizing it as a sign of the system being broken. No matter which system you have, it's not feasible to have elections every week. There will always be bad actors that will require an election to remove from power.

Coalition governments build support by appealing to particular interests of the various party members. That means an “Abolish the National Debt” Party and a “Green New Deal Party” are going to form a different kind of government than a “Green New Deal” and a “Small Business Alliance” party.

Exactly the problem. I don't have a say in the nature of the coalition that's formed after the election. I'm not going to 100% agree with any party, and in a Prop Rep system the policies will be determined after the election during backroom deals to form a coalition. I'm in Canada and the Green Party basically imploded over Israel-Palestine even though there's no chance for them to ever have any influence over foreign policy. Many times I might agree with a party in theory, but politicians tend to be whacky people and party leadership tends to be even whackier. But since the 2 MPs they have represent their communities they can do that job even when the party leadership goes batshit crazy. People can still call their Green Party MPs and those MPs can bring up those concerns in the Parliament even when the party itself is completely broken.

The only reason why Justin Trudeau is PM is because his party has built a lot of capability in identifying community leaders and recruiting those people into the party. People may not even like the party but they like the person they have running in their area, so a few seats can be picked up in this way. It's interesting how bringing in community leaders is a good strategy to win an election in the "bad" First Past the Post system isn't it? In a prop rep system you'd want to fill your party with yes men who would go along with whatever the party leadership wants.

Also compare what happens if the party leadership goes nuts in both of these systems. In the first past the post system, if a majority of members (who are beholden to their communities) thinks the leadership is bad, then the leadership is gone. In a prop rep system is there's any members that don't like the leadership those members get replaced, because the seats belong to the party, not the people that sit in them.

See politics isn't just a numbers game. There's debate and discussion and compromise. Power dynamics should be the primary consideration in any system. Prop Rep is a party first system, the power flows down from the party leadership. In a community representation system the power flows up from the communities. Voters decide who represents the community, community leaders decide who the party leader is. No system is without flaws, but a prop rep is completely dependent on parties which creates too many disconnects between the voters and those in power.