they just used the old command form, German is an ever changing language, colloqually and technically through the spelling reforms
By the way, this sentence is not a common German expression. You can say "durchs Feuer gehen", which pretty much matches the English "going through fire and water".
Or you can go "through thick and thin" which would be "durch dick und dünn", with the same meaning in English and German.
In my language, we say 'go through dead bodies' in the meaning do everything (also illegal) to get what you want. Is it something like that?
No it means that I am moving all of my stuff out of my apartment in 1 day no matter how impossible it is just to get in done in 1 day. Not necessarily hardship, but someone that's high strung and bossy and wants things done immediately no matter what even if there's no emergency.