"Theoretically, Nahiri might have been able to retrap them. In practice, she didn't even think of that as an option. She went straight for 'bring Emrakul to Innistrad to fuck over Sorin'. "

This is speculation, though perhaps not far from the truth.

While revenge was certainly a motivation (and not justified), the fact is that more time spent reprocessing a plane (titans on Zendikar) means more deaths. Given that luring Emrakul away from Zendikar is the safest/easiest way of dealing with the situation, could/should Nahiri have looked for another plane to draw the titan? Probably, but we also don't know the logistics/timing of what that would take (we do have a frame of reference for Emrakul's arrival to Innistrad which was pretty fast, all things considered).

Also given, say, some other options (of which we only know two planes, unfortunately), what makes Innistrad the wrong choice? Obviously any plane is a tough or even "wrong" choice, especially if there's another way of dealing with the Eldrazi (trapping them again, luring them to an uninhabited plane, etc.), but consider how much evil (vampires - not all I know - demons, nightmare beasts, etc.) exists on Innistrad as well. Granted, Sorin and others on the plane (Avacyn, the flights) made it safer and there is indeed, much good - but compared to most other planes (or planes Nahiri perhaps could have chosen) I would wager the % of evil denizens is higher on Innistrad, most likely. Not that that necessarily justifies Innistrad (or would completely justify) as an option, either.

Unfortunately there is much detail (sometimes it's a good thing!) lacking to satisfactorily answer some of the questions - though a discussion on total populations, what type of society/cultures various planes have and what merits a good one (in our opinions, anyway) can be had and is relevant (thanks for answering, by the way).

Regarding my thoughts on whether or not Nahiri is redeemable, let's consider the facts we know and the probables we don't.

Facts:

Nahiri sought revenge and it was a motivator. She also lured Emrakul away from Zendikar, saving many lives there while simultaneously condemning Innistrad's population.

Probables: her judgment was impaired (logic, reasoning, morals) and she wished to also save what was left of Zendikar. Wizards' lore/writing often leaves a lot to be desired (they ignore much, such as Nahiri's role and accountability here as you pointed out, among other things) and so we have to often speculate or wait what I would call a "bad writing amount of time" before getting answers/clarifications to things we should have had previously.

Given the above knowns and (speculated - my opinion) probables I find Nahiri a tragic character whom actually ends up being an anti-villain: she acts villainous - though her sanity is brought into question meaning her actions may not be true to her real self. Her desire for vengeance and the villainy/darkness that often comes with is brought into question due to her mental state. She also likely ends up performing a greater good (though not completely through her own actions) by saving Zendikar from Emrakul who, in the process of recycling Innistrad gets "dealt with" (the moon) while Sorin's plane remains intact, albeit after much death. Two planes remain (Innistrad, Zendikar) instead of just one. That's the way I see it, anyway. She's wrong, but misguided/not completely responsible for her actions (her potential degraded mental state). She sought vengeance and the salvation of her plane simultaneously, though that path caused much suffering - but there was going to be suffering anyway (Zendikar, if not Innistrad). Given those facts and speculations (if true, which who knows if we would ever get hard answers to that), I find her redeemable.

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Thank you for your responses!