I think any serious presidential competitor has yet to emerge - none of the Communist candidates, or Navalny, are credible IMO. 9 months is plenty of time to make a campaign happen though.

It's at least as likely that, off the back of the poor performance in the war, and especially the dismal reaction to the Wagner affair, that Putin will be simply "encouraged" behind the scenes to retire rather than run again, and United Russia will put forward some kind of "interim" candidate who will probably win.

FWIW if you want examples of leadership changes during a war, how about Neville Chamberlain? Or the two revolutions in Russia itself in 1917?

By what mechanism?

Putin is up for re-election in March 2024. Presidential term limits may have been removed in 2020, but he still needs to go back to the polls. If he cannot be sure of a resounding victory, there's a good chance he will retire rather than seek re-election. He may try to declare martial law and suspend the constitution, but his political capital to do that is a lot more meagre than it was in February 2022. More than any other election since he was first elected in 2000, he will need real support from his political base, and I'm no longer convinced he will get it.

Prigozhin is now in a pretty good position to be kingmaker, even if he won't or can't be a presidential candidate himself.

Of course everyone with any loyalty to the state, or with any future political ambitions of their own has come out in support - at least publicly - of the status quo. To do otherwise would be political suicide. But that doesn't mean they actually support Putin privately.

The manner in which a coup is stopped makes all the difference. It only enhances the position of the government if the government is seen to deal with the coup attempt promptly and effectively. If the government is not seen to deal with it effectively, then even if the coup is ultimately unsuccessful, it undermines the government.

In this case, Putin was not seen to deal with the coup effectively. Wagner was able to secure the Southern Military District headquarters - the main operational HQ of the war in Ukraine - in Rostov-on-Don with essentially no opposition, then drive most of the way to Moscow with only minor skirmishes along the way. If we believe the widely-disseminated reports, then seven Russian military aircraft were lost to Wagner actions, but only one Wagner truck was lost in return. Putin was not seen or heard from for a long time, and reports surfaced that he had flown to St. Petersburg, and when he did appear, his message was not particularly decisive. When the coup was abandoned, it was done not through force, or by Putin publicly facing down Wagner like Yeltsin did in 1991. It was abandoned due to some back-room deal, brokered by Lukashenko of all people, the details of which have not been made public.

The Russian government's internal security apparatus appears incompetent because it did not consider Wagner a threat, even though Prigozhin had been telegraphing his intentions for days (and probably planning it for months). The Russian military appears weak and overstretched, because it could not protect its operational HQ by force. Putin appears weak because he disappeared at the crucial moments, and large parts of Russia appeared sympathetic to Prigozhin even if they weren't prepared to directly take part. The image of Putin being the supreme ruler in charge of everything looks pretty suspect at this point, and Russians know it because most of the events happened live on Russian state TV.

Putin has a long history of wriggling out of difficult situations, and he might still pull things out of the bag, but I think this is the beginning of the end for him.

The only party responsible for the war is Russia. If you invade another sovereign state with the express intention of toppling their government and installing your own, war is what you're going to get.