Kiritan's explanation of the fact Iomedae is a nascent god is just that - Lawful Good isn't good enough. Its gods are ancient and alien. Trustworthy, but - trustworthy to not hurt you, not trustworthy to leverage you towards fixing everything wrong for everyone who has ever lived or ever will. Hell still exists. So do Abaddon and the Abyss. It's unacceptable, and everyone knows it's unacceptable, and Iomedae is the one person who might be careful enough and powerful enough to change it. She and Aroden have obviously been angling that way for - well, for as long as Kiritan has personally been going to Aroden's church in Absalom and hearing about the Crusade. Iomedae understands Good, can transmit it to other people, embodies Law and can inspire it in her army. She can do things no one else can do, and Aroden hasn't picked a new herald since Arazni died because it's obviously Iomedae, and she took Urgir by miracle and she thinks like a god and she does magic only gods can do and she's explained why there needs to be a new Lawful Good god and the world needs her desperately. She'll go for the Starstone when the time is right but she's - already the person who will.
(The Starstone? Oh, it's a trial Aroden set up that makes you a god if you pass. Obviously anyone can try it if they want. Mostly they'll die, though.)
Tiaves's explanation is slightly different. "Look," he says. "Iomedae is a legendary hero, the sort of person whose abilities can't be approximated by imagining anyone else, or an army of anyone elses. She's a tactical genius, she's not at all a bad politician, she goes around wearing say an eighth of the accumulated wealth of Golarion's largest empire, she has fought Tar-Baphon hand to hand a dozen times and it's him who generally has to run. Nearly every other powerful person on the planet has served on her crusade, or is the target of it. She just took a city that hadn't fallen for thousands of years. She is obviously greatly favored by Aroden, and extraordinarily powerful, and once we beat Tar-Baphon, if we beat Tar-Baphon, she'll have her own country in what used to be Ustalav, headquartered in a ludicrously well-defended dwarven sky-citadel with two different permanent active Miracles defending it. She'll have a large and experienced army, and no enemies on Golarion who'd dare get within range of her.
A lot of people ask themselves, what's she doing next, right? Some kinds of person, it'd be 'build an empire', but she has treaties already, with all of her new nation's neighbors. It's the undead we were asked to take up swords against, not the living. She means the adventurers presently in her service to disperse, in some cases all the way to Tian Xia, with stories of this war and the miracles she worked in fighting it. She has accumulated immense influence, respect, admiration, and, frankly, worship on Golarion, and it's not so she can conquer, and it's not so she can retire. She's not - none of us are - the kind of people who retire.
The work she's been at, for at least ten years, is building a church, and building a legend, so that when she goes for the Starstone a lot of people across Golarion will worship her as a god already, and reach out to her in prayer, and that's helpful to gods that are just starting out. If the war ends well, and ends cleanly, then within a few years, she'll set up the habitable parts of Ustalav as an independent country under her church - that kind of thing is easier to do as a mortal - she'll disperse the word across the world, she'll write her holy book and have it cried on the streets of every city, and she'll go for it. There are no guarantees, but - Aroden built the trials, and Aroden works, through her, the greatest miracles anyone has ever seen Him work, and all of Good will be in her favor, and Gorum too, and Pharasma who hates the undead, and all of Law save Asmodeus and Zon-Kuthon who know She'll be eternally their enemy - she's been building a coalition among the gods just like she's been building one among the mortals, is what I'm saying. It would not surprise me to learn she picked the crusade in the first place to get Pharasma to back her ascension.
And there's no one in the world I'd rather have bring human values, and what it takes to see them realized, to Lawful Good. I've scoured the history books and there's no one in history I'd expect to do better. She's smart, and good, and careful, and she's ours, and She'll be our god and we'll be her church, and then I guess we'll fix everything, everywhere, for everyone."