@flyingtonight The inappropriate passengers should be moved BEFORE taxying. I don’t think you can wait for an emergency situation to arise and then politely explain that the unsuitable passengers should be moved to be replaced by more able bodied folk. The crew might be dead!
An emergency exit, blocked by incapable pax, is not a good idea.
I have also mentioned to the crew that there were inappropriate pax in the em exit seats on BA recently. Handled well….by moving the pax to Club Europe! The purser explained later that it happens frequently because of online check-in. It should be the gate staff who should notice a possible conflict before boarding.