Great post Kevloaf... I love a hypothetical discussion!
I think that we can (almost) safely rule out Don Cossack. IF the scenario above were to happen, you would have a number of solid players with a good prep behind them and one prep run wouldn't be enough.
Again, in the above scenario, I can't see any of the others competing with IN-FORM Cue Card, Thistlecrack, Coneygree and Djakadam. (Unless Douvan is re-routed, but I think that this is unlikely)
I'm not a fan of Djakadam (can't justify why!) - I just think that he could win a sub-par Gold Cup (2014 esque) BUT in the above case - this would be anything but.
If we ignore GC Trends, then my hunch would be Cue Card. He would have had an uninterupted 2 seasons, he is proven at this level, has solid form in the book and whilst Coneygree will be closerthan when they last met - I would still side with Cue Card (as a safer option).
The unknown is Thistlecrack, but if the 2017 Gold Cup were to be his first decent test in open company - I would side with Cue Card