Toranaga knew he sent her on a crash dummy mission, which then begs the question, why all the political manoeuvring if a battle is whats gonna decide everyones fate. Might aswell just have a show of battle after battle. I do hope they've something up their sleeve to tie all this political stuff in
Because if he went head up he'd get destroyed. It was stated MANY times that he didn't have the largest army, and that was cut in half during the earthquake in Izu. This is why he even reached out to his brother in the first place. And that plan fell apart.
So he needed a way to erode the chokehold Ishido had on the other council members and their armies. Now the vibes in Osaka are all fukked, because it's going to be widely know that Ishido had a noblewoman killed for simply wanting to fulfil her master's wishes which is a HUGE violation of their customs. Especially for something as innocuous as just leaving.
He's no longer going to have their military support, even if they are confined to the castle. So now Toranaga can actually fight on level footing or even have the advantage because Ishido will have to be wary of both the attack AND the knives at his back.
As I said in a previous post. The whole show Toranaga is using their cultural norms and expectations against them. The ONLY one that peeped it was the Portuguese dude, because subterfuge/betrayal is more of a European thing. It's also why everyone things Yabushige is an idiot, because every time he THINKS he's making a savvy move he ends up in a much more perilous position or the exact opposite of his intent goes down.
If Mariko woulda committed seppuku, it would have been DIRECTLY because Ishido didn't let her do her job basically. But what ended up happening is MUCH worse, because now multiple people saw him give her clearance IN PERSON, and a mere hours later she's blown to bits in her death robe (which should have tipped us off because, you ONLY wear that kind of kimono when you are dead or about to die and she kept it on even after Ishido stopped her).
It's a MASSIVE miscalculation.