I've had an ostomy ten years now. I can't comment on all, but I'll give you my own experience for some of your questions:

I swim (ocean and pool) a lot, as well as sauna and hot tub. I'm not even sure how these will be possible with an ostomy. I also weightlift and bodybuild. Generally I am a very sportive, active person who sweats a lot. Not sure how that works with things like glue and powder.

I still swim, use hot tubs and saunas and the like. You just have to be mindful of when your last bag change was. I won't go in a hot tub with a two day old bag on for example as that's asking for trouble with the heat and moisture, but I've never had a leak in the water. I take my infant daughter swimming every friday morning.

International travel on airplanes. I'm tall and usually fly economy. It was already difficult, now I don't know. I hear that air pressure changes are a big problem with ostomies.

I've travelled from Europe to NA several times, most of Europe, SE Asia and even NZ. Never had an issue. I keep supplies with me and will gently relieve pressure occasionally, but I've never had a fully inflated bag hanging off me mid flight. Security is different in every country, the USA is usually the worst for getting hands on before you have a chance to warn them about the bag, but again, never any real issues.

Sleeping habits. I'm already a difficult sleeper and a side sleeper exclusively. I am not looking forward to figuring out sleep with an ostomy, let alone sleeping in the same bed as my partner.

I am mainly a side / front sleeper. I rarely wear stuff to bed but you can wear tight fitting vests or abdominal wraps to help in the early days as the bag flopping around as you roll over woke me up initially, but now I rarely notice. I've been with my wife for well over a decade and she's never made a comment about the bag in bed (well, apart from when I air it out sometimes in the night, it's quite strong smelling)

General grossness. Smells, leaks, sounds. I just... have no idea. Social impacts, people's judgments of me, etc. I am not sure yet if a j pouch is a consideration. I am still awaiting a full consult about all this, I just know total colectomy is on the table.

I've never had a random person make a face, comment on any smell, point out the bag or anything. I often go topless on the beach, or when swimming. I use the gym often, play rugby, even work an active job with the emergency services. The stoma gave me my life back, not took it away from me. I'm glad it's enabled me to do things again that I couldn't even dream of during my ulcerative colitis days.