There is no such thing as absolute margins so I have no idea what you mean here? Margins are just margins. 
Again this makes no sense. If you don’t want the container white then don;t give it a background colour and then its background will be whatever the body background is? I don’t see how negative margins has anything to do with this question either?
You seem to be throwing tons of ill advised rules at this project where none are needed. The only complicated aspect is the automatic columns which these days is achieved easily with css columns as I have already shown. If you want to manually finish a column then as Sam suggests you can use the display:inline-block approach on a div on each column but then you don;t have automatic columns unlike the method I have used.
Here’s a full example with borders colours and table at the bottom and looks exactly like your drawing.
This is the sort of valid code you should be using along with the correct doctype. Even if this is not exactly what you want you need to work along the same approach and not throw unnecessary rules at the layout. Most of all you need to ensure that what you put on to the page is semantic html. Double or treble breaks are unreliable and semantically very bad and should never be used. I realise you may be limited by your editor but that doesn’t mean I will endorse bad practices