Did they have a shared 455?

Odd.

One of the cops says: "We can't stand here with our thumb up our áss.."

Now, wait a moment or two.

First of all, isn't it usually "sit", instead of "stand"? (Although logically, it'd be easier to stand in such a situation than sit)

Second of all, do they all have a shared thumb and shared áss? Did they buy a donkey or something? And what's this mutually owned thumb?

"I can't sit here with my thumb up my ass" seems correct.

"We can't stand here with our thumb up our ass" seems incorrect and full of 'wrong', as they say.

Who wrote this line? And why did he/she write it in such an illogical way?

They have multiple thumbs, and there are multiple asses. He uses the word "we", which refers to multiple people.

Why is there only ONE thumb and ONE áss in his sentiment, though?

And why does the captain (or whatever he is) say "Simon says.." instead of "The threat was" or even "Simon said"?

Just so that it would sound identical to the "Simon says" game? No one would say it like that in real life, for crying out loud.