Q: Once Webb is no longer functioning, will it remain a relic in its L2 orbit?
Dr. Hammel: After the @NASAWebb mission is ended, it (like WMAP, Herschel, and other L2-based telescopes before it) will stay in its L2 orbit, as L2 is a gravitationally stable location.
Q: Assuming all systems and components perform optimally and the telescope isn’t disabled by some external force, what are the most likely failures that will result in retiring the mission? What's the rosiest (unofficial) estimate for how long the mission could last?
Dr. Hammel: @NASAWebb's life-limiting factor is hydrazine for station-keeping at L2. Our prime mission is ~5 years and we hope to get to over ~10 years with careful management https://jwst.nasa.gov/content/about/faqs/faq.html#howlong
Q: Will ground based telescopes be able to detect the reflection from Webb’s backside as it orbits around second Lagrange point?
Dr. Hammel: Sure, if the telescope is big enough. @NASAWebb will be around 17th magnitude, so folks with access to telescopes over 1-m may detect it as a point source. Here's an article (paywall, sorry) about a telescope network that was tracking GAIA at L2
Q: I understand the images from Webb will be comparable in sharpness to Hubble--and see further back in time and in more detail--but how would you compare Webb’s expected images to those of prior generations of infrared space telescopes--such as Spitzer, Herschel and WISE? And of those, is there catalogue of images we should look to in calibrating our expectations of the kinds (aesthetic) of images Webb is expected to produce?
Dr. Hammel: You can expect images with a similar aesthetic as you see for Hubble, as many of the same people will be involved in press-release processing. So check out https://hubblesite.org
Dr. Hammel: Also, Webb has a much larger aperture than Spitzer, so it will produce images with much better spatial resolution. Here's a set of Spitzer images - imagine these with better resolution and the Hubble aesthetic :-)
Dr. Hammel: Finally, here's a simulation of Webb image of source compared to a Spitzer image of the same 