Dolly737 the flap 15 requirement for SE landing is to satisfy min GA gradient requirements. So long as your satisfyed the runway is CLEAR and you HAVE been cleared to land, there is no reason to reconfigure from flap 40 to 15 (which in so doing will screw you up so late in the approach). Retain your landing flap, increase the operating engine power to maintain your desired speed and drop in some rudder. Its not a difficult exercise.
There is no hard and fast magic altitude that dictates a go around/landing decision due to engine power loss or fire. but certainley anything sudden below 500 ft AGL in IMC or VMC requires prompt decision-making. On every approach a good pilot knows his chances of 'getting in' or not. If theres ANY doubt (fluctuating vis/cloud at the minimum) a GA would be initiated in this instance ASAP to allow for altitude loss during flap retraction (theres no way youll climb at flap40!) and the subsequent SE GA.
We operate into many short runways where flap 40 dictates the limiting landing weight. Anything less than 40 and youll go through the fence.