Yes, in this situation you can have your cake and eat it, too. If you are able to successfully change your itinerary to get on the DTW connection (maybe using FlyNow just after midnight or something), your flights *should* post with what you flew - LAX-DTW-TYS.

It's odd that your outbound posted with the DTW connection. Curious...did you happen to check-in online for those flights before you realized traffic would be bad and you couldn't make those flights? That's the only reason I can see those flights posting. Even then, you actually flew the MEM connecting flights which means your BP was scanned and they should have posted. I'm guessing, though can't confirm, that your account was originally posted with TYS-MEM-LAX but was later manually corrected. Did you happen to check your account the day or two days after the flight?

-RM