Who could replace Joe Biden as the 2024 Democratic nominee?

I suppose some, if not most, of you were not around for the last DNC "open convention," which was in 1968. It was a mess.

So, what options do we have now? We can hope the convention does the right thing and select a person that can defeat Trump. IMO, our very existence as a free nation depends on his defeat. We must unite behind the selected candidate. Only then can prevail. I believe VP Harris is the logical choice, but I will support whoever gets the nomination.

I think if the DNC want any chance at all in floating a viable candidate, they need to make friends with RFK Jr.

I'm pretty sure that's been decided in that who will replace Biden. It will be Harris.
You ask good questions, albeit a little late. Make no mistake, people have been noting, commenting on, and pointing to the many examples of Biden's decline, even before he was elected. To quote what you quoted;

"in the coming days, the Party will undertake a transparent and orderly process to move forward as a united Democratic Party with a candidate who can defeat Donald Trump in November. This process will be governed by established rules and procedures of the Party. Our delegates are prepared to take seriously their responsibility in swiftly delivering a candidate to the American people."

Does anyone really believe this? For three years, despite the calls mostly from the right to have Biden checked out, his staff and the media have been willing accomplices in telling the American people "He's fine." Now, say what you will about Trump, but that is all irrelevant. You (we) have been lied to in an unacceptable manner. We don't know who's really been running the country and thus the "elected" President has essentially been a puppet. I'd like to know who? That would be transparency...

As the DNC moves closer to the convention, I have little doubt they will continue to lie, to tell you Harris is the best candidate ever, and that she walks on water. It's all desperate BS, all orchestrated because they knew they could not get Biden across the finish line to continue using him as a puppet. My prediction for now (while noting there are 3 months to go in this election) and pending any significant surprises, Harris will fail, and she will fail hard. This is based on what we've already observed from her. She is no more (less actually) articulate than Trump (and believe me, I do not think of him as some great orator by any stretch of the imagination), she's accomplished nothing as VP, to include failing to serve as the border Zahr (which was the first task Biden gave her).

Now, with all that said, I do not expect those who vote left and/or solid Democrats to vote for Trump. However, there is no energy for her and there likely won't be any. It will take energy to overcome what will likely be ~80-85 million people voting for Trump. The DNC is rightfully in a bad place in which they need to come to terms with and resolve. Since 2016, they have been pushing more hate than I thought imaginable, and this is still their platform now. This is why people are trying to kill Congressmen, Justices, and a former President running for office again. The DNC needs to tone it down, but they only want to do it on their terms, because for the moment, it's the only tool they think they have to fight Trump and the GOP. What they fail to realize, is that it was them that has driven people like me more and more to Trump. Not because I like him, but because I can't help but respect him for taking on so much of their shit and beating them on the things that truly matter (such as these bogus court cases).

Politics aside, the American people don't want to see their courts abused for political reasons. They also want consistency in the application of law and justice, for our leaders to actually enforce the law properly, and for the campaigns of hate to end (be it against one man or the party that chooses to vote for him).