I'm not entirely sure which parts exactly you're referring to for the "last choices", but from what I can tell these are all either shortcomings of the 'banjax' or things that were not in the power of the twins.
First, the banjax (the chemical that victims need to ingest before their souls can be extracted). At the end when Marinus reveals that she laid a trap for the twins she says this:
Of all the shortcomings in your operandi, your 'banjax' is the most antiquated. Truly! An anima-abortifacient so fragile that unless the patient imbibes it of his or her own conscious volition it fails to work—we haven't seen the Shaded Way deploy such a primitive formula for fifty or sixty years. What were you thinking, Grayers? If you'd only updated it you could have injected it into my body just now. Or tried to, anyway.
So, the first part of the problem is that the twins are using "outdated technology", so to say, which means that one of the choices the victims need to make is to ingest the right food or drink in the orison. I don't think there's any more detail on why exactly the banjax has this problem, but we can just assume that's the way it is.
This already explains why the twins can't just kidnap them right away once they enter the aperture. Norah's viewpoint also makes this fairly explicit when she's trying to lure Marinus into the building, where she's afraid that Marinus might just leave again:
We're at a delicate stage. I need to coax Marinus up to the house without her either sensing a trap, [...] or being spooked into running for the exit.
It's also not possible to "suasion" (i.e. mindcontrol) the victims into taking the banjax because they're engifted:
If I push the banjax too hard, she'll grow suspicious. [...] If Marinus weren't so engifted, I could have just suasioned her to eat it; but then if she weren't so engifted, her soul would be useless to us and she wouldn't be here.
I'm not sure whether the portraits really have any purpose and whether the victims also must reach the attic by themselves (which I think they all do), but these might be part of it as well.
I think this pretty much explains most of the choices you see. If there are any particular instances I overlooked that you had mind, please let me know in a comment. I'll try to go through the ones I can think of:
It seems that in this case the banjax is the coffee Nathan gets from his dad. Here, I'm not quite sure why the damson Nathan eats almost right away when sitting down with Jonah isn't the banjax. Having to reach the attic first might be part of it.
Here I'm pretty sure that the banjax is in the meal that Norah serves him on his second visit. Why they chose an orison that had to make him revisit, I can't say, and also why they kept the orison going, although once again it seems to end with him in the attic.
This is the one I'm least sure about. She seems to eat and drink quite a lot during the party. My theory is that one of the brownie piles has the banjax which would explain why Jonah (in the form of Melbourne Mike) shows up while she's choosing. I see two possibilities here:
- She picks the wrong one, which is why the orison keeps going. They give her the escape sequence with Todd (suasioned by Jonah) to make her trust him unconditionally. The moment when she actually gets the banjax would then be the point where Todd kisses her. This would also explain why Jonah puts so much effort into wooing her (as Todd) throughout the orison. I'm not too sure about this though because it would be the only case where the banjax is being imbibed without eating or drinking something.
Alternatively, she might have picked the right brownie pile and ingested the banjax then. The reason for continuing the orison would then be the Grayer twins (particularly Jonah) just wanting to put on a show, being slightly arrogant. These quotes (said by Jonah) from the last section would support this:
Your and Todd's thrilling bid for freedom was another part of the rat's maze we had you run through, an orison inside an orison. A suborison.
I am speechless with admiration sister. [...] If the academy awarded Oscars for Best Orison, you'd be a shoo-in. Truly, it was a masterpiece. [...]
In either case, there's no indication she actually would have been able to flee at that point.
Of course, this chapter has the only other real chance to escape prematurely which is when the aperture appears inside the house and Sally peers out and has the option to flee. It's made clear by Norah that this was a fault in the orison and was not supposed to happen:
And the aperture—appearing of its own accord like that, and open. The girl nearly bolted.
For this one, it's explicitly stated that the first banjax is the tomato juice which she ignores because it doesn't look very appetising. They then give it another try with the Cashew nuts later on which works (and is pretty much at the end of the orison).
Yes, yes, sister, our guest turned up her nose at the tomato juice, but we banjaxed her good and proper with the cashew nuts.
Seeing how she fails to be able to leave the floor she's on after eating them, I don't think she has the option to leave the orison at that point. She doesn't ever reach the attic by herself (since the orison isn't even in Slade House), but maybe she was brought there right away before the orison started when she was brought into the house by the Blackwatermen.