Two major things had already happened to upset the old balance of power before the story even opens. One was the rising influence of Nemesis, the other the technological revolution.
Assuming for a moment that Nemesis is some sort of behind-everything boogeyman rather than a Fae-specific infection (which I do not actually believe, because there is no need for it, it does not explain anything not already solidly explained), it's still a specific individual disrupting influence. So that's an argument for attacking Nemesis, not for attacking the Red Court. If anything, it's an argument for allying with the Red Court against Nemesis; they are visibly not benefiting from its actions either.
In recent decades, it's become possible to travel, communicate, and otherwise operate around the world in a way that the Wizards have great difficulty coping with.
The ability of wizards to mess with technology does go a fair ways to counter that, and Harry has been able to contain that to some extent when he has tried, so I have a reasonable amount of faith that more skilled and experienced wizards would be able to do better.
This made the White Council vulnerable, but the Senior Council and the rest of the older Wizards were not fully aware of how much that changed everything.
You buy into Harry's prejudices on this front, then ?
They had not waited centuries for the right moment, they waited at most a few years, 20 tops. Harry was a handy pretext, but absent him something else would have worked.
We see what happens when the Red Court trespass on Winter in DB (they must have, to attack the Council in the Ways, because the Council only ave access to Ways in Winter); they get a through hammering in PG as a direct result of Faerie actions.
We also see what happens in Changes when they make a false peace offer; they get exterminated.
We see, in SG, Mab acting to punish Nicodemus for his violation of truce with the Archive in SmF.
The evidence does not come down in favour of Accords violations being trivial things to get away with. Therfore the Red Court absolutely need that
casus belli.
And as for being the aggrieved party, all they have to do is set up some situation where a Council member faces an intolerable choice, it's not that hard to do. Harry was just convenient.
I'm inclined to think any loyal member of the White Council would be expected to have sacrificed one life to prevent a war. It very much seems to me that that is what Bianca expects; she does not read as suicidal.
How does that save Susan?
It doesn't. Why is Susan's life worth more than the tens of thousands the war has taken, and why should anyone other than Harry find his choice there acceptable?
Ortega had already set things up in such a way (or been manipulated to set things up in such a way, the difference is moot) that there was no way to achieve such an alliance. It wasn't one of the options available.
Believing that requires disregarding Susan's take on Ortega's motivations at the start of DM, yes ?