So essentially my dilemma is do I pay more for a phone that feels more pleasant to hold, but *might* have downsides that don’t become apparent until my return window is closed. And for that reason I’m quite tempted to keep my 16 Pro for another year.
I have been following your angst with interest as I have many of the same questions coming from a 16 PM. I will wade in with my decision tree. Mind you this is ALL theoretical as I don't own an Air and you get what you pay for (in other words I am like 95% of the people with opinions here (and that is being generous), but I am honest enough to admit it).
Performance/overheating : At one point you say the PM wins the specs war. Yep. Except of course convenience form factor. But there have been folks saying for quite a while the A series performance outstrips the actual need by quite a lot (i.e. these monster computers aren't being fully utilized). The only folks that seem to disagree are gamers. Are you a gamer? If yes get the Pro, if not, keep thinking.
Btw, being a gamer is not the same as playing games. I play games all the time, but they are not GPU and CPU intense, they are basically turn based. I have never had one throttle my phone. The you tubers claim there are some action games that throttle even the Pros. They highlight them when trying to show a phone overheating. I do know my Pro would get hot, because the 16 series is known for that. Apple has theoretically addressed this in the 17 pro, but data is still being collected. I doubt (but what do I really know) the Air will heat more than the 16 PM.
So my own thoughts are that the Air performance will be fine FOR ME. BTW, I do find it interesting you never hear from other groups on performance issues (photographers etc), but just gamers. Not sure if that means there are no issues or they are more shy.
Photography: You are right, we don't know how much the lack of an ultra wide is going to impact us. Your angst seems to be, 'but will I miss it later somewhere down the line?' The good news is, you can do a retrospective analysis on what you have now and find out now rather than later. Scroll through your photos you have taken over the years. Pick out your favorites, the ones you really like, look at the exif data and see what lens it used. There may even be apps that numerically analyze that for you, tell you how many pictures with what lens. Lightroom classic does that for my 'real camera.' You will find edge cases, the odd picture here and there taken with the ultra wide or the telephoto but I bet you will find the majority of the pics you like are with the standard lens and the ones you really like are too. For me that is certainly the case, but admittedly for whale watching you want a telephoto and I bring my real camera with real telephoto lens. Thats hard to do with any camera phone. Majestic landscapes, sure wide angle is nice, but even with my real camera if I am hiking and dont want to carry 4 lenses, a standard lens and taking panoramas pics to stitch together works fine. And this is easier to do on an iPhone with the standard lens..
What I am saying is just use your history to tell you how you use your camera as a predictor of your future.
Battery: this one is more personal. We all use our phones differently, no one can speculate on what works for you (though plenty at MR will try lol). But for me, I can count on one hand the number of times my phone has gotten low to the point I was worried I would miss out. And that is over almost twenty years of use since the original iPhone. One hand. And I have NEVER ran out. On transpacific flights I do carry a battery, but these days most planes have USB outlets. So even then... I am good at battery management. I dont wait to make sure my phone is charged. It's not a worry, it is an easy thing to do, especially with wireless. When sitting at work, put it on a charger. When going to sleep, put it on a charger. When in the car, I use CarPlay, it is charging. You get the idea. Is that you too?
Now my adult sons, routinely, and I mean at least weekly, run out of power. They are always scrambling looking for a charger. They don't seem to think about charging until they need to charge. maybe it's a generational thing. they dont seem concerned about taking care of things, maintenance etc, I view battery charging as maintenance.
if you have rarely or never run out of power with your existing phones (some with far less power than the current air), battery life may not bite you in the future. Again a look at history approach.
Having said all that, FOMO is strong. It is hard to put down the PM and try something different. But in the end, it is only money (yeah I know), how much will you really lose if you keep it, and decide halfway through the year it really isn't for you and sell it and trade up? Is that worth the price of trying something new? Only you know. No phone decision is permanent.
My strategy btw, is doing what you are doing, in a sense. I am currently in a remote part of the country (log cabin in the woods), the nearest Apple Store is 200 miles from here. When I go home, I will go to an Apple Store and pick one up in the store and see what I think, then make my decision. I don't think the air will be a novelty thing, if I like something, I tend to keep liking it.