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 <PERSON> 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, <PERSON>? 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So, the first part of the problem is that the twins are using "outdated technology", so to say, which means that one of the choices the victims need to make is to ingest the right food or drink in the orison.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. <PERSON>'s viewpoint also makes this fairly explicit when she's trying to lure <PERSON> into the building, where she's afraid that <PERSON> might just leave again:
We're at a delicate stage. I need to coax <PERSON> 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 <PERSON> 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:
<PERSON>
It seems that in this case the banjax is the coffee <PERSON> gets from his dad. Here, I'm not quite sure why the damson <PERSON> eats almost right away when sitting down with <PERSON> isn't the banjax. 624
Having to reach the attic first might be part of it.
<PERSON>
Here I'm pretty sure that the banjax is in the meal that <PERSON> 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.
<PERSON>
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 <PERSON> (in the form of <PERSON>) shows up while she's choosing. I see two possibilities here:
1. She picks the wrong one, which is why the orison keeps going. They give her the escape sequence with <PERSON> (suasioned by <PERSON>) to make her trust him unconditionally. The moment when she actually gets the banjax would then be the point where <PERSON> kisses her. This would also explain why <PERSON> puts so much effort into wooing her (as <PERSON>) 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.
2. Alternatively, she might have picked the right brownie pile and ingested the banjax then. The reason for continuing the orison would then be the <PERSON> twins (particularly <PERSON>) just wanting to put on a show, being slightly arrogant. These quotes (said by <PERSON>) from the last section would support this:
Your and <PERSON>'s thrilling bid for freedom was another part of the rat's maze we had you run through, an orison inside an orison. 624