In my 200A ITE main panel there's a feed to a 100-amp sub-panel that’s elsewhere in the basement, via a 100-amp breaker at the bottom of the main panel. The photo shows the wire (#1 AL, I think) and the connection to the breaker. As you can see, the neutral for the subpanel feed -- circled in red -- is not connected to the neutral bar in the main panel but to a lug on the enclosure. Seems to me this is a poor idea because the neutral current from any unbalanced load in the subpanel is now being carried by the enclosure. It's been working this way since before we moved in 20 years ago but I only noticed it this year.
My thought to address this is to move the subpanel neutral to a lug attached to the neutral bus. I do have the right lug (Siemens brand) and it will accommodate the wire. I have to make sure the lug fits in the physical space between the neutral bus and the panel cover, and that I can bend the wire up there, but I'm pretty sure it will work. Does this sound like the right approach? And is it worth the trouble, or is the whole issue of neutral current flowing in the enclosure not as big a deal as it seems to me?
Related question, as you can see this subpanel feed is at the very bottom of the main panel, which is fairly large (14 spaces on each side). Should it be higher up so it doesn't load up as much of the bus? Or is that not important?
(Edited to add photo of 100A feed entering subpanel)