It's already November, and the(yet) mosquitoes are surprisingly still around. It's November, and (yet) mosquitoes are surprisingly still around.

There was one in my room, which woke me up in the middle of the night. There was one in my room, which woke me up in the middle of the night.

I couldn't go back to sleep and thus I'm writing this to pass time. I couldn't go back to sleep and thus I'm writing this to pass time.

I couldn't go back to sleep thusso instead, I'm writing this to pass (the) time. I couldn't go back to sleep so instead, I'm writing this to pass (the) time.

"thus" feels kind of weird here, maybe it's just to me but it sounds more natural to say "so" or "so instead". "the" is optional, again just makes it sound more natural to me

I pity them because every single meal is a livfe or dieath situation for them. I pity them because every single meal is a life or death situation for them.

Sounds more natural

You'dThey either get slapped and killed or be unnoticed and get away. They either get slapped and killed or be unnoticed and get away.

You can use the second person I guess, but it's gives off the impression of a more literary style, as if you were talking directly to the mosquito. I think it's a cool style though!

You'dThey either get slappwatted and killed or be unnoticed and getfly away. They either get swatted and killed or be unnoticed and fly away.

You swat a fly or bug. slapping sounds odd here and "get away" feels like something is attacking them, if they were unnoticed they wouldn't be attacked so simply just saying "fly away" is fine

I left my door open for a bit and now ithey seems to be gone. I left my door open for a bit and now it seems to be gone.

In this sentence, you're referring to the one mosquito from earlier. "They seem" could also be taken as singular but only when you don't know the gender of a person, you just use "it" as a pronoun for animals. Using "they" as a pronoun for a mosquito feels a little like you're personifying it.