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.

I don't think this is realistic. You're asking for sweeping massive electoral pattern changes that won't happen without a massive crisis functioning as a catalyst. I can't see a source for such a crisis on the horizon though, the frontline isn't going to change while the economic sanctions were resoundingly defeated and hurt we european residents significantly more than Russians.

FWIW if you want examples of leadership changes during a war, how about Neville Chamberlain?

Chamberlain was not changed by an election of the population of the UK, he was changed by Conservative party infighting leading to the 1922 committee demanding his resignation in a "do it or we'll do it for you" ultimatum, as they have always done. Chamberlain resigned his position as leader of the Conservative party and Churchill took it.

There was no election in the UK between the years of 1935 and 1945.

Or the two revolutions in Russia itself in 1917?

Generated by an unpopular war in a series of wars that the tsar repeatedly got people killed in for his vanity. This is not an unpopular war though and there is no anti-war movement, it is supported by every political faction of the country. Even the euro liberals don't openly state their opposition and that's not because they would disappear (they wouldn't) but because it would be wildly unpopular and harm their political growth, they are forced into silence through the conditions that currently exist.

that Putin will be simply “encouraged” behind the scenes to retire rather than run again

This requires factional fighting, which is non-existent at this time. He has broad across the spectrum support both in his party and in opposition groups because they all see him as ushering in a multi-polar world, which is extremely beneficial to the interests of every faction that exists. I still do not see where you think the factionalism exists for this to happen. Who? Why? What faction is going to push him out and for what purpose? With everyone wanting to see the completion of this project there's no faction internally to interrupt it.