I liked Part #3. Made me feel sorry for Myra, but well, she gave a show. The only improvement I could think of is a little bit more detail about these final cuts that made her change her mind and beg for death, as well as how it felt for her and what went on in her mind during these crucial moments of breaking.

Regarding part #4 I first thought about him overpowering Aiva and killing her, but then again , maybe it's better if he fails spectacularly, trying.

I think we'd need a confession. Aiva will want to know what exactly happened to her mother and why. I'm sure she's a good listener, and has ample methods to have him talk while he's bound.

I could see her devising some mean "I torture you until you kill yourself" setup.
He'd be bound, with a blade mounted under his throat. Really all he needs to do is cut his own throat and die. She'd make sure life is agonizing enough, maybe with some mean things to his genitals.

But Donovan is the type that would try to fight free instead, it would be fun if he managed to spill some of Aiva's guts and slice her open. However mortally wounded himself and not going to make it.

It would be a fitting end if they are both staring at each other, knowing that they are likely both going to die, with not much more to say between them. What would he do? What would she do?