Yeah, there were indeed many typos... Thanks!
I think you mean these sentences:
"An alien vessel crew abducts a human, brainwashes them"
"organism is ready for insertion into a human being, which would turn them"
"I lent some assistance with the interrogation of our recent captive. Actually, I used the grenade-in-mouth trick to convince them to talk"
I can't see anything wrong with these sentences. What is the singular/plural problem again?
Sorry for being unclear. You're right, these are the sentences I was talking about. I'll try to explain. "a vessel crew abducts a human" - you use the singular number in the first part, but then you changed the number and used the plural: "brainwashes them". It should be "abducts humans and brainwashes them" or "abducts a human and brainwashes him" to keep consistency.
"organism is ready for insertion into a human being, which would turn them" - the same thing. It should be "into human beings, which would turn them" or "into a human being, which would turn him"
"I lent some assistance with the interrogation of our recent captiv
e." - in the first part of the sentence there was only one captive, but in the second part there are already a few of them: "Actually, I used the grenade-in-mouth trick to convince
them to talk". Sorry for being a bore. I know it's not that important.