You're overthinking it. Just buy the ticket from Delta.com and don't worry about it.

Airlines look for habitual hidden-city ticket users, nested ticket users (wherever that's still relevant), or perhaps other things that frequently refunding paid F tickets or whatnot.

Even if the airline for a brief moment thought you were a habitual throwaway ticket user (another thing that's not as relevant anymore), the very fact that you bought a later ticket on the route in question demonstrates that you weren't seeking to get around a tariff rule.

In short, it's a nonissue and you don't even need to hide the behavior. Don't use Orbitz or another airline, unless you actually want to. Provide your FF# and earn your miles. Even among those here who feel hidden city is an ethical grey area, this particular flight is not.