[dmurray]

Perhaps playing 1. e4 2. Bc4 3. Qh5 4. Qf7 (and resigning or offering a draw if some move isn't legal) would minmax this further

The problem isn't really well defined. Elo rating is assumed to be determinable independent of what opponents you face, so scoring 50% against opponents rated 1800 gives you the same information as scoring 26% against opponents rated 2000. In practice that's obviously not completely true, and for degenerate examples like the ones we are discussing it completely falls apart.