I accept that things can seem better when they are, in fact, worse and your example of the neophyte with the equaliser just happens to have been me some 30 or so years ago :-)
I do think that we need to distinguish between things seeming better when they are, in fact, worse when the cause for this is simple ignorance and an unskilled ear and cases such as the comb filter artifact examples you mention. There's 2 very different things going on in these examples and we need to avoid confusing them.
Whatever the mechanism for the room lens' effects, it is most definitely sensitive to placement and orientation. In that sense, they are 'tuning devices' just like an antenna is a tuning device—stick them in the wrong place and/or point them the wrong way and both will give you bad results. Put them in a right place pointed the right way and you get good results. One thing that variability in results with the room lens suggests to me is that they are genuinely doing something. If they weren't and we were simply fooling ourselves, one would probably expect the results to be more consistent regardless of placement.
I don't think I accept your view that measurements will distinguish whether the sound is made 'truly better' or whether it's psychological. We're in this hobby for pleasure and to some degree I tend to equate 'truly better', which is after all a value judgement and a different assessment than 'truly accurate', with the consistent provision of increased enjoyment. The psychoacoustic aspects can't be separated out from the rest of what we hear. What we measure is not what we hear, only the parameters of the soundfield falling on our ears. If we interpret the measurements in the light of what we know about psychoacoustics and we have sufficient knowledge of the relevant aspects of psychoacoustics affecting our perception in a particular case, then the measurements and our experience should correlate. If they don't, then something has been left out of the equation and that something could be missing knowledge, an ignorant or uneducated ear (your neophyte with the equaliser), a personal preference, or it could even be one of those nasty psychological mechanisms which occasionally fool us into believing that black is white.
I don't think this is one of those cases often, and erroneously, called a placebo effect. There are simple and well established mechanisms like reflection, difraction, and the Helmholtz resonance mechanism which should lead us to expect physical acoustical effects from the room lens, and their sensitivity to placement and orientation is consistent with those mechanisms. I'm satisfied that we can reasonably assume the existence of audible physical acoustic effects so the only question remaining about the value judgement people make about the favourability of those effects is whether that judgement is due to ignorance or not. Explaining why the effect is perceived as 'better' or 'worse' is a different matter and firmly in the realm of psychoacoustics rather than physical acoustics.
David Aiken