No responsible parent will allow a child on a balance bike (or one with training wheels) in any scenario where they'll 'endanger' anything.
Balance bikes teach you to ride a bike (maintain balance while steering). Training wheels teach you to pedal a bike.
Learning balancing and steering first, and then learning to actually cycle the pedals is the right order of events I think.
I have googled and the quick search tells me that the gap is between 6 to 12 month.
2. I have never ever in my life seen even one balance bike with brakes. So as soon as a kid can "glide" on it, he is a danger to himself and to the people around.
Does learning balance a bit earlier outweigh the danger? I don't know.
As an adult I try to walk as far as possible from kids on these balance bikes, unless parent is holding their hand or handlebar. Getting hit at a significant speed in the leg is not fun.
2. A very quick google search got me this¹. More than half the balance bikes on there have brakes. I've seen similar on other bike websites. I agree it's irresponsible to not have brakes, especially for the bigger kid models.
I have a 7 month old now. I want to get her on a balance bike before the end of the year, or maybe start with a trike. Exclusively indoors, or under very strict supervision from me if we are anywhere outside. As soon as she's big enough, she'll get the smallest "big" bike where I have removed the pedals, but we have the front and back brakes present. Then she'll continue balance-biking on there till she's ready for the pedals.
Oh please. We're talking about a 1 year old on a 6lb bike going jogging speed.
The benefit in learning curve is huge. I didn't have the benefit of balance bikes, so had to learn to bike at ~6 years old on a big pedal bike and struggled for a long while to balance it.
In contrast my kid learned to balance on a balance bike at age one, and had a seamless transition to a pedal bike at three.
Training wheels are the worst for multiple reasons. The kid doesn't learn any balancing since the bike stays upright on the training wheels no matter what they do. And a pedal bike with training wheels is quite heavy (for a 1 year old) so it's more frustrating.
The best way to learn to cycle is to start on a balance bike when very young (under two) which gets the balancing part hardwired in the young brain. Then it is an easy step up to a pedal bike when they are a bit older and can handle the extra weight.
We also lived in an apartment and a tiny balance bike was a lot less disruptive than a pedal bike with training wheels.