It needs a major design change switching engines on an aeroplane. For example, the RR engine with 3 spools is heavier than the GE and P&W equivalents. The mountings are different, the thrustlines are different, the whole interaction of the engines on the wing change, the ducted airflow supplies, hydraulics and electrics are different, and then you have to carry out all the development flying and manual and performance manual testing! Big Job. I don't think there was enough of a delay by RR to make swapping engines worthwhile.
As for the MD11 duct, I don't think Douglas thought that there was any effective gain in efficiency to pay for the major design change.
So, out of interest, why the questions, or are we just chewing the cud to pass the time?