It's not entirely clear what Polly plans to do with the blackmail information. Obviously it's something, and not something small like getting her squad out of trouble or setting Jackrum up with a favor.
Polly's expression as she stared at the wall would have frightened a number of important people.
They would've been even more concerned by the fact that she spent the next several hours writing things down.... She copied out the entire notebook.... She wrote a few letters.
On the one hand Polly is not happy about how things have been going in Borogravia.
The same old stuff. They were recruiting again. The same old words. The same old croakings of long-dead soldiers, inviting the living to join them. General Froc might be female, but he was also, as Blouse would've said, "a bit of an old woman". Either that or the heaviness of those epaulettes had weighed her down.
... All we were given was a chance, thought Polly. No miracle, no rescue, no magic. Just a chance.
But on the other hand, the conflict she's going away to join at the end is being pushed by Prince Heinrich of Zlobenia.
"The swede-eatersh is at it again!" [Gummy Abbens] said. "They're gonna invade 'cos of the prince saysh we belong to him now."
"It's all down to him being the Duchess's distant cousin," said Polly's father.
"But I'd heard it still wasn't settled!" said Polly. "Anyway, there's still a truce!"
"Sheems like he'sh shettling it," said Gummy.
Polly is of course no friend to Zlobenia in general or (especially) Heinrich in particular; their last interaction concluded with her threatening to kick him in the groin (again). So presumably she's going off to fight him... one way or another.
How many ways can you fight a war? Polly wondered.... I know a man who writes things down. The world turns. Plucky little countries seeking self-determination... could be useful to big countries with plans of their own.
Taken at face value, this implies that she's planning to drag Ankh-Morpork into things by using her connection to William de Worde and the Times (whose calling card she is staring at). Commander Vimes previously implied that, all things being equal, AM would rather not let Zlobenia conquer Borogravia.
"Look, we'd rather Prince Heinrich wasn't the ruler of two countries. That's make one quite big country, much bigger than the other ones around here. So it'd probably get bigger still. He wants to be like Ankh-Morpork, you see. But what he means is he wants power and influence. He doesn't want to earn them, he doesn't want to grow into them or learn the hard way how to use them. He just wants them."
Similarly, her stated reason for seeking out Blouse is:
"He knows about the clacks. He knows about other ways war can be fought. With intelligence, for one thing."
But then that doesn't explain what all the blackmail is for, unless she's planning to use the sensationalist story of all the women in Borogravia's army to rouse Morporkian support. It's possible that - Borogravia being a proud country - she foresees the need to "persuade" them to accept Ankh-Morpork's help when it's offered. It's also possible that her two missions are just that: two separate missions. She's going to use Ankh-Morpork to save her country, and in return for that gift, she's going to demand change, using the blackmail when inevitably the high command gives her trouble. And maybe she has no plan (only a "very firm intention", as Vimes might put it) and is simply stockpiling anything that might help her.
As for the two young women she meets on the ferry, ultimately it comes down to what Vimes muses about in Night Watch: she chooses to see this as her job, and it's in front of her. Regardless of whether or not she can bring the war to an end politically, there's still battles to be fought, and regardless of her feelings about the Borogravian government and high command, she's still going to look after the common soldiers the way Jackrum did for her and many others.