It is hard for me to separate imaging and soundstage as well. In part because I don’t really use the latter term much. There are alot of different ways you can think about this stuff though. And I don’t say that mine is necessarily more correct than some others.
When it comes to the quality of the stereo imaging though, I think everything can potentially get into the act (to use an apt Floyd-ism). So yes, the clarity of the image is important. The precision of the image is important. The spaciousness, width, depth, height (in some cases), and general immersiveness are all important. And imo, these can all potentially encompass more than just your basic FR.
But even if it is mostly related to FR, I think it does a bit of a disservice to think of it just in those somewhat narrow terms, because the quality of the stereo image can potentially be affected or imapacted by so many different things in a headphone’s and transducer’s design and usage.
The openness of a headphone, for example, isn’t really related to FR. I think most would agree though that open headphones tend to image better than closed headphones.
Symmetry is certainly related to FR. But you can’t look at a single FR curve for a headphone, and really get much of an idea of its symmetry or imaging just from that. To really get an idea of a headphone’s symmetry, you have to look at both the left and the right channels, and compare the differences in their FR and relative amplitude, which is not a trivial thing.
The symmetry you hear in a headphone can also be affected by other components in your audio setup. Especially the amp, which may tend to drift a bit to the left or right at different volumes. So if you want good symmetry (and imaging), then you also have to pay some attention to what’s going on in your other components, particularly in terms of their left-right stereo balance. But also in terms of noise, distortion and impedance, for the best results.
If your headphones don’t image well to begin with though, then they probably aren’t very good headphones! And the most you can probably do to try to fix it is maybe add some EQ, which has potential upsides and downsides of its own. That is how important good imaging is though, imho.
It really is sort of the whole ball of wax, when it comes to sound quality in headphones. And it’s what separates an average or mediocre headphone from a really good one, imho.