You have certainly done your homework. I have the same size wrist and had an IWC pilot watch (39mm Spitfire UTC) for about 6 years before selling it (which I sometimes regret). I will try to answer your questions.
1. I'm sure the watch can withstand crutch use, as the movement has a shock absorber system. The only thing I would perhaps consider is using a stronger spring bar (assuming you go strap instead of bracelet), as I would be concerned about extra wrist pressure, though this may not be an issue.
2. This of course depends on whether you go strap or bracelet and how tight you wear your watch. The bracelet will add quite a bit of weight and will slide down more if not worn tightly, which could be annoying.
3. Though I much prefer straps, particularly on pilot watches, IWC makes some of the best bracelets in the business. But the bracelet will add significantly to the price and weight of the watch. Plus with straps you can mix and match and give the watch different looks. I have over a dozen straps of all varieties and colors and love switching them up.
4. The manufacture vs. ETA movement is always a tough call. The ETA 2892-A2 is an excellent workhorse that incorporates some features that IWC originally designed into the movement. For this type of watch I would have no problem with it. My grail watch is the Ingenieur 3239 which uses the same movement and is even more expensive (which is why I don't have it yet). Keep in mind that servicing costs can be very high with Richemont, but any competent watchmaker can service the 2892. On the other hand, there is an undeniable allure to a manufacture movement, notwithstanding the higher acquisition and service costs. Unfortunately, IWC does not make a movement small enough to fit into a case that our wrist size can handle. That will change over the next couple of years, as IWC recently announced some new movements, one of which is smaller caliber (42110) designed for its entry-level watches.
I understand and appreciate your desire to buy new, but I echo the sentiment of Toothbras regarding an older Mark version, particularly the 39mm Mark XVI. If you can find a NOS (new old stock) Mark XVI, I think you should get it. Or even a used one in mint shape. The Mark XVII is a great watch, but I like the XVI better in terms of size and design. My 6.25 wrist handles the 41mm XVII fine, but my wrist is flat, and even so the 41mm is really pushing it for me. If your wrist is more round, the size could be an issue. You really need to try it on for size.
Regarding IWC and the military, I am unaware of any current official tie. Of course, IWC's work with the St. Exupery foundation is admirable and an interesting pilot watch connection. And as one of the handful of companies that supplied the original pilot watches to the Luftwaffe, the historic military connection is authentic.
Finally, you mentioned that you want this to be a long-term watch. Perhaps the red triangle and the triple date window will get tiresome after a while. Whereas I doubt the simple and classic look of the Mark XVI will ever get old. To me that watch is like a perfectly fitting navy blue blazer--it will never be out of style. Just something to think about.
Good luck with your decision, and I hope it works out.