Siuan didn't have to be secretive and manipulative as she was just to conceal her knowledge of what Moiraine was up to with Rand, but she was busy "preparing" (read: Manipulating) world leaders into accepting him while being just as careful to make sure no one in the Tower knew what she was up to despite making it obvious to EVERYONE she was up to something. Even if she didn't reveal anything to anyone she could've remained more involved and engaged with Tower leaders than she had to have been for one dissident White and a long absent Red to engineer a coup requiring most of the Hall. They were ripe for her overthrow because they'd been reduced to isolated spectators (and often not even that) and lackeys in a game whose goals and rules only Siuan knew.

As for setting Egwene up as Latrae, that may well be, but how much she did to "save the world" remains to be seen. HER plan was to use create the Choedan Kal to reimprison the DO; there's no evidence that was even possible, and had the Shadow known the access keys were within territory they captured it could've been disastrous. All we really know was that the full implementation of LTTs plan required women and was thus significantly impaired without them (though is his comments in ToM indicate he hadn't anticipated and consequently planned for the DOs response anyway, so in that sense perhaps Latrae did save the world). However, returning to what I consider an overarching theme of the series, within the context of LTT vs. Latrae it seems clear that HE was the one acting preemptively and unilaterally and Latrae was a check on that. The current timeline may represent a role reversal in that Rand/LTT is generally a lot more inclusive and open to dissent than Egwene (with the notable exception of the Seals, but most relevant parties agree with him there anyway, and he's still stated his intent to make an ARGUMENT, which Egwene has already committed to rejecting in much the way that Latrae rejected LTTs strike at Shayol Ghul).

In ToM Brandon merely continued and developed Egwenes plotline in the way it was already heading. I'd have to think him a very clumsy and tactless writer to believe he portrayed Egwene as unbearably as she appears in the last book, and I don't. If I didn't consider her narrative value a lot higher in the scenario I described I probably wouldn't support it.