Isn't it possible to add turns, etc. to Inverted Impulse Coasters?
Yes. I think what he was getting at is that the Intamin invert track should be triple rail and impulse track is quad rail, so if you wanted to build something like Tornado, a new track type would be preferable.
existing track types with missing (useful) pieces.
That's pretty straightforward to do at this point* and not really what I was getting at. I found out that there are actually a handful of free slots so it would be technically feasible to do a completely new track without a new save format, so far as I can tell, and I was thinking of trying to do it. But it sounds like it's unlikely that such a change would be accepted upstream, so I probably won't actually attempt it except to see if it can actually be done.
* The programming is straightforward. Creating the required sprites isn't, which is why I gave up trying to do it.
It seems you had most of the track pieces made in at least some state.
Actually I only had a tiny fraction of the track pieces made. I still have those, and I've been experimenting with them, but I can't really make anything close to an RMC layout with what I have. I did experiment briefly with an Intamin invert: