Hard to comment without seeing it but, if the boiler is showing signs of corrosion and starting to become problematic then he is giving her good advice imo. Whether I agree with the choice of replacement boiler or not is a different matter.
External vessel is a good idea as only a 10l inside your boiler so probably undersized for your system. One of the main underlying causes of boiler breakdowns, undersized or incorrectly charged expansion vessel, often overlooked by installers.
You can position the microswitch so that the pin is visible then view it from underneath. As somebody has already said, also make sure that the plunger is still attached to the end of the pin.
Can't see any other profession that's willing to stick their neck on the block just to save someone a few quid or a little inconvenience, so why should any rgi?