Would it make more sense to land on the inners and depart on the outers

[procede]

The Lden noise contours (with the same traffic) will be almost identical with mixed mode vs. closely spaced parallels. ATC will surely prefer segregated runways, as workload will be lower and capacity more constant, but of course it is more expensive for the airport. Also during maintenance still one runway will be available.

For Heathrow mixed mode means the same amounts of flights overhead per day per runway, thus the same Lden contour, but during the whole day instead of just concentrated in the morning or afternoon.