I don' think your enjoyment of other Truffaut films will be spoilt by them following in the lines of Shoot The Piano Player. I think it stands apart from his other films, although JetJ might be as experimental but in a very different way. Silken Skin is a more "on rails" French romance, and probably could have done with some of STPP's verve.
That picture of Aznavour & DuBois looking kind of vacant, misses the vital element that whilst they appear nonchalant deliberately, Charlie is thinking at 100 mph (in somebody else's voice), which we only get to hear occasionally. This may be an example of inconsistent tone, and had that experimental edge, but it highlights one of the greatnesses of the film. Truffaut throws a "kitchen sink" of technique into the film, but a very large percentage of it hits its target; but that, of course, is just my opinion. That seems to be a defining factor in new wave, that the pace and tone and pov and etc etc will switch and change constantly. This is the only one I have seen that works to any great extent. So to that end it shows the potential for greatness in the style.
However, when you get to Day For Night, Truffaut's confidence in the style, means he can inject many different styles but only in service of the story, without the feeling of experimentation. Perhaps, you could comment on what you like in DfN that resonates with this earlier effort, if you feel there are any significant points of comparison.
Apart from that, the point you make about borrowing from noir films. Honestly, so what? Genre is a platform, it doesn't hurt any film, just because you can define the genre it comes from. My only problem with defining noir as a genre, is separating great ones like Blood Simple, Brick & Winter's Bone from their progenitors, just because they aren't B&W or old enough.