Whilst a considerable amount has been written about archival preservation and deterioration of film stock Frank, because of their commercial and archive storage importance, much of it has been to do with motion picture film and microfilm. Not so much that I know of, which directly answers your question. (Which is not to say that it has not been documented of course, only that I am not currently aware of it.) For the short answer, I should say first of all that, from a practical point of view and assuming normal darkroom use, I have never heard of negatives fading from exposure in the enlarger.
However, you don't specify whether you are referring to conventional silver negatives or to chromogenic negatives, which as you know are based on colour film technology and contain dyes, they do not contain silver. These two materials are different.
Henry Wilhelm's seminal book 'The Permanence and Care of Color Photographs: Traditional and Digital Color Prints, Color Negatives, Slides, and Motion Pictures' examines (amongst other things) the fading of colour film as a result of 'visible light and UV radiation by the image dye molecules, causing them to break down', but this is in relation to the intense exposure involved in motion picture projection. He also states that it is the light rather than heat that causes the dyes to fade (see the second part of your question). These conditions are very different from the exposures given in the darkroom of course, but we do know that colour transparency film fades with repeated projection.
I have therefore asked my friends at Ilford/Harman Technology if they are aware of any reports of fading of their chromogenic Black and White film 'XP2 Super', as a result of enlarger exposures. The short answer was no, but the question has been passed to their technical department and if there is anything further to report from them, I will let you know. They were also unaware of this problem occurring with silver based negatives exposed in the darkroom, and again I will let you know if there is any further news from their technical department on this.
During the 1980's The Image Permanence Institute in Rochester, NY, conducted work on silver based microfilm and ways in which its permanence might be improved. In their paper 'Stability of Black-and-White Photographic Images, with Special Reference to Microfilm' authors Reilly, Nishimura, Cupriks, and Adelstein described the mechanisms of fading with silver, either as improper processing or oxidising chemicals, the driving forces being moisture, air and pollutants (notably gases) - not light alone - and they examined the protective benefits of selenium, gold and sulfiding. Their paper makes interesting reading, for although it doesn't directly answer your question, it suggests the mechanisms for silver fading lie in other areas to those you are asking about, but more importantly, it underlines the importance of careful processing and protection - which I am sure outweigh any theoretical risk from enlarging exposures.
I will let you know if I find any further information on your question.