That's not how batteries work... You can't just drop a camera, add another component that draws more power, then call it even. If you don't use the third camera, it's not drawing extra power when it's not in use. There's no harm in including it other than a marginal cost and added weight. But part of the "iPhone Pro" moniker means that all three cameras are included. And honestly, if I had to pick two cameras, the 3x telephoto is far more useful than the ultrawide. I currently have a 13 mini and a 13 Pro here with me, and I can tell you that since getting the 13 Pro I've used the telephoto far more than the ultrawide.
I know Apple thinks they're upselling people by gatekeeping the telephoto lens at the Pro level, as opposed to before when it used to be included with the X and XS. But until they offer a Pro mini it's missing the mark. It's not about the cost to me -- I'd gladly pay a Pro Max price for a Pro mini. It's 100% about the size and weight of the only devices that include that particular lens.
As far as the display goes, the ProMotion display uses less power because it can scale the refresh rate dynamically all the way down to 1 Hz and match refresh rates for video content, which are often either 24 Hz or 30 Hz. Most time spent on the phone is looking at static or near-static content. The 13 mini and regular 13 run at 60 Hz whenever the display is on, even if it is displaying static content. That wastes a lot more power than occasionally ramping up to 120 Hz for a fraction of a second while the system animates an app opening or closing.
When the minis first came out I wanted the stainless steel for its appearance. Having since used a 13 Pro for a few months, I'd actually prefer the aluminum. I don't care for the appearance as much, but it is significantly lighter -- so much so that it's the difference between holding a heavy phone that is uncomfortable and a phone that feels just right.