I agree that Detroit's first option isn't to trade Drummond, but I also wouldn't label him untouchable (like A Davis). Their situation reminds me a little of LAL back when Kobe and Shaq were fighting. They could have either kept Shaq and let Kobe walk or sign Kobe and trade Shaq. By trading him they were able to gain additional pieces to go along with a re-signed Kobe.
With Monroe as a RFA and Josh Smith having so little value, they could really cash in on Drummond if they, a) wanted to compete now and, b) thought that a Monroe, Smith front court pairing was fairly ideal.
I would leave Houston out of the deal since I don't think they benefit too much from it, but a package if Rondo, Green, and picks isn't bad at all and it would actually balance out their team quite well.