I’ll check in tomorrow (or I’ll be checking in tomorrow) and I’ll check out on the 12th.

Yes

The second option sounds more natural and would be better as “consider it payback for helping me last night.”

#2 is the most common phrasing. #3 can be used if you are emphasizing a specific bed. #1 is not used (“in bathroom”)

Yes, both ways are commonly used.

“You know what” is just a way to get someone’s attention before you proceed with whatever you want to tell them. -You know what? My mom said my brother isn’t coming to Christmas after all -You know what? I knew their plan would never work -You know what? Sarah and Bill broke up The possibilities are endless.

Resident bad cop refers to the good cop/bad cop technique. Calling himself the resident bad cop means he always plays the tough guy role. Onions are my own is a play on the disclaimer “opinions are my own” which means you are representing only yourself and not your employer or any other group.

There was no stopping / no preventing