Mecabq let me see if I can try and answer some your questions.

For the NW/KL codeshare agreement they have 100% anti-trust immunity so they can and do share revenue across the Atlantic and coordinate schedules and slap each others codes on most every flight the other operates that has even the smallest potential for a codeshare. Without their anti-trust immunity they would not be doing this. They are in fact the two airlines that are the closest to one another across the North Atlantic.

For your question about DL/AF ex IST. Most agreements are set up so that the non-operating airline (in this case DL) gets a certain number of seats on the flight and once they are gone, they are gone. How the revenue gets shared I am not sure about.

For the US/UA question it is like this basically to connect 2nd tier cities that the other airline doesn't serve and to connect to international flights. One might fly CID-ORD-CLT-ILM on a mixed of metal but every flight might have either a UA or US code and lets UA say they serve ILM and US serve CID (if they dropped their service, can't remember for sure but you get the idea).

Hope this helps some. ^