Well, would you rather have a vehicle with lackluster sales = sales at all, or no vehicle = no sales? I'd think since the tooling is already paid for and being used, having a Mercury TX (or SX) would be better than nothing.
As you said, I don't believe a Mercury Edge/MKX would be a good idea, too much overlap, and that's probably why one wasn't designed.
You mentioned bringing Euro models here....as I've said many times, that'd only be a short term solution, and one that is increasingly less worth it. With Ford globalizing it's products, etc., bringing Euro Fords over would just put Mercury in the same rebadge boat it's in now in a few years....and with the globalization already beginning, the new global Fords are only a year or two away (some models at least), thus rendering a Euro Ford -> US Mercury rebadge useless at this time and a waste of money.