Ditzyboy, I completely disagree with the statement you made saying
"Once the lady is sitting there the cabin crew don't really have many options to move her. Cabin crew are not doctors or physio and hold not qualification or authority to tell the lady she is not capable of lifting the exit or performing the function"
We are not doctors but we do have the authority to tell a pax that we do not CONSIDER they are able to perform the function required. Admittedly I do generally find another way of moving them (the trick you mentioned of offering them another seat so they can keep their handbaggage works well) but if necessary will state the fact that IN MY OPINION they are not suitable.
Also, I think in the US things are a bit more stringent. Last year I was told that I would have to move from the overwing exit as I was too small to sit there. I politely advised the crew that I was cabin crew and operated this type of aircraft and had opened the exit on an annual basis for the last 8 years ... still had to move but did so without complaining.