This is all going to be a matter of what they can do and what they will do. Technically, what the Diamond desk told you is correct.

Now if you get to DTW, and you go to the CS desk and tell them you suddenly need to get to CVG, and you do not have bags checked through to SEA, they may reroute you, but there is no way to guarantee this.

This is where that oft-used acronym YMMV comes into play.

I also think it might matter whether you are on the paid fare or the certificate fare. If you are the companion, they are probably less likely to be accommodating than if you actually are the one who technically paid for your seat. I could be wrong (I get surprised by the problem solvers on here all the time), but I'm not sure anyone can guarantee you success if you do not fly the full itinerary.