Never wore a helmet until one day in 1993, when I was test-riding a bike (Bridgestone XO3 with the mustache bar) I was considering getting, and the bike shop had just that day instituted a policy that all test riders had to wear a helmet. I grumbled while they fit the loaner, and took the bike out. I was still grumbling to myself about it as I was coming down a hill about 15 minutes later, when a guy in a Subaru wagon passed me too close and hooked the end of my left handlebar in the gap between the body and the tailgate, causing the handlebar and the front wheel to immediately spin almost 90 degrees to the right, and arresting the bike's forward progress down the hill; my progress wasn't arrested at all, and I somersaulted over the handlebars, landing on my back and slamming the back of my head into the pavement - or would have, if the helmet hadn't taken the impact. While lying there and watching the bike doing its own somersault (seemingly in slow-mo) above me and wondering whether or not it was going to land on me (it missed), I decided that wearing a helmet was a really good idea. The damage to me was mostly minor other than some nerve damage in one hand that healed in about six weeks, although the bike was worse for wear.
I did wind up getting an XO3, although not that one and had it for a couple of years before it was stolen out of the back of my Subie wagon (never occurred to me that I needed to lock the bike while it was inside the car, and someone broke the back window and yanked it out). I always wear a helmet now, except when making a long slow climb when it's warm as I overheat easily. In that case it's strapped onto the back of my fanny pack, to be re-donned as soon as I reach the top. OTOH, there've been studies that show that drivers tend to pass people wearing helmets more closely, as they consider them more responsible riders and thus less likely to ride erratically; one of the classic studies (in the U.K.) found that looking female (the researcher put on a long blonde wig) also increased the passing distance, presumably for the same reason as not wearing a helmet.
As for bells and horns, I find my voice works fine, and shouts of "don't do it" or "watch it" have prevented many a case of dooring, right hooking etc. Overtaking other bikes, a quiet "passing on your left" still works, although less experienced riders can be startled by it and swerve, so I say it before getting next to them. A bell is probably less startling, and sounds less threatening to some; a loud horn may cause them to crash.
I really should get a mirror, as although I ride a flat-bar performance hybrid now (a used steel-frame Peugeot probably dating from the '90s; for some reason, all my used bikes have been French, two Peugeots and my first road bike back in the '70s, a Mercier) rather than a road bike with drop bars, checking behind me is still a bit of a chore compared to an even more upright riding position. I'm not a big fan of handlebar-end mounted mirrors, seeing them as a potential source of injury, but lean towards the helmet or glass-frame mounted variety. I'm a fan of steel frames for commuting/urban use, as I prefer their ride and the extra weight isn't a big deal for shorter rides.
I've been meaning to replace the flat-bar with a mustache, or at least add some bar-end extensions. but inertia has prevented that to date. I consider the mustache bar the best all-around type for commuting and shorter weekend rides (up to say 30 miles), giving multiple hand positions and allowing you to ride upright in traffic, more aerodynamically when you want to push yourself, helped by a shift in the brake and shift levers from closer to the the bar ends to the forward side of the curve - my 1993 XO-3 also had fake brake levers on the forward side, which gave you even more hand positions. For those who don't have a clue as to what a mustache bar is, this one's pretty close to the shape of the one on my XO-3:
https://www.niagaracycle.com/catego...l_ms=9032009&gclid=CMvU_dX5hNUCFUtNfgodO_gEFw