If the visual turns you inside the OM make it the "from" point...again two or three key strokes.
Depends on the airline. Air Canada, for example, doesn't allow route mods in the Boeing below 10,000 feet from what I'm told.
And, using the primitive text based user interface in the 75, do you really want the PNF punching buttons inside the marker? Maybe the PNF was heads down trying to do what you suggested when the miscue occurred.
A non-GPS 75 probably wouldn't be able to accurately tell you were lined up on the taxiway instead of the runway until it was too late, especially with the course up display in a crosswind. You have to mentally rotate the picture on the HSI to try to make it match what you see out the window with a crosswind component. The dotted line would have enough error from map shift to make you disregard a minor displacement I would think.