Do high and narrow seat and back bending provide good bike riding posture?

You don't have to do any of these things to commute. If you value safety over speed I'd recommend a more upright bike (Dutch style) which helps improve visibility and makes it easier to look around. They also allow you to ride in the clothes you want to wear at your destination. What you call old-school is current every-day practice in a lot of Europe.

1. You could lower your saddle so your feet can touch the floor but this isn't an ideal riding position usually. What would be more ideal is learning how to stop and set-off properly. You need to hop out of your saddle as you come to a stop and put your non-dominant foot down. Then use your dominant foot to put its pedal near to front and top of the stroke. When you set off you push down with your dominant foot on the pedal and at the same time lift yourself back into the saddle.

2. The saddle should support your sit-bones (at the bottom of the pelvis) which in men aren't all that far apart, hence the narrow saddles you see. However a hunched over riding position also means less weight is put on the saddle and more on the hands and feet than when using a more upright bike. Wider saddles can be more comfortable when you have more weight on the saddle but can be detrimental on longer rides. Your saddle type may depend on your commute length. Generally cheaper squidgy saddles are made to entice people by looking comfortable but are actually worse than a firmer, narrower saddle which is designed to support your sit bones properly.

You also mention riding in the debris-strewn area of the road. Another advantage of Dutch bikes is that they're designed to be robust, with heavier, wider, reinforced tyres.

I would recommend looking into various techniques for positioning yourself in the road, but I can't offer advice on the US since the streets are designed very differently from Europe. Hopefully there are US-specific resources out there.

Edit: I forgot to mention mirrors. I recently changed bike and my new one came with a mirror mounted on the end of the handlebars. It's helped enormously with being aware of my surroundings and traffic, so I'd thoroughly recommend one. Presumably a helmet-mounted mirror would be the same.

I was wondering how I can stop safely and easily without landing one or ideally both of my feet on the ground

As thospor already wrote, the standard technique is to not sit on the saddle but instead stand up in the pedals, so you can sink down to the much lower top tube. Practice this a bit on its own (with low saddle), until you feel comfortable moving around your body anywhere over the bike. Also practice bringing the pedals in any position needed. (The quickest way, provided you don't have a coaster brake, is to turn the pedals backwards.) Three most important positions to have ready: ● left foot down ● right foot down ● both feet level, strong foot forward.
To get off the bike, you then just assume one of the foot-down positions, unclip/remove the other foot from the pedal, and step down so the bike's between your legs, saddle behind your back. I recommend practicing to do this with either foot just in case, but normally it's best to leave the strong foot on the pedal (or put it there whilst standing stopped) to quicker get going again.

That said, I personally almost never use this technique because it's kind of awkward. Depending on the situation I either

• Stop next to a curb, so I can just put my right foot down on it without getting out of the saddle.
• Stop next to some lantern-post, wall etc. and lean on it with either my handlebars or shoulder.
• Push down my dropper post, so either foot can easily reach the ground. This is equivalent to adjusting the saddle to way down like you currently have it, but modern mountainbikes allow this adjustment whilst riding, at the press of a button from the handlebars. Dropper posts are super useful, but seem to be still largely unknown outside of mountainbiking.
• “Fall sideways” and land on the corresponding foot.
• Or just straight away step off the bike entirely.

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some people bend their backs forward when riding bikes, just like in Olympic bicycle competitions

Your concern is not without reason. What's harmful for the back is however not so much the bending itself (unless it's all too excessive) but having load on it whilst bent. There should be very little load on the back when riding in a good balanced setting, but it requires some trial&error to find the right saddle and handlebar position. My back does start hurting a bit when I'm riding for a longer distance with a backpack, the saddle too far back, and/or the feet not clipped to the pedals. (Clips help because, I think, they allow the legs to work “differentially” without transmitting the force through the body.)