DL's COC doesn't affect your travel on other carriers. Besides, like you said, they can't regulate them.

What they can do is invalidate your DL ticket if they find out you bought a ticket on another carrier to circumvent Delta's fare rules.

They don't care if you're breaking NW's COC with a back-to-back. That's NW's problem. Of course, with NW, it's the same thing -- you violate NW's COC with your NW ticket but not DL's.

Put the two together, and you have two "illegal" tickets.

As to whether this applies to travel back to the origin other than restricted roundtrip fares on an airline (e.g., unrestricted air fare, train, bus, car, etc.), I don't know. Interpreting the COC literally, it does say "two or more restricted round trip fares". The spirit of the COC might include all travel, not just restricted air fares, but that's getting a little ridiculous in the interpretation IMHO.

Wow, and we haven't even gotten into the ethics yet, just trying to interpret the fare rule and get the names of the schemes right.