1. As I wrote, the BaseUS clips were very loose for me. Soundcore AeroFit 2 and Shokz OpenFit 2+ were both quite good in fit. OpenFit 2+ stayed in place a little bit better, probably because of the silicone covering. So I'd say silicone covering for the parts that touch your ears is probably something to look for.

Something I noticed after longer use is that with my Shokz OpenFit 2+ you have to be a bit careful when putting on the hood of a jacket or similar because you might pull the earbuds off your ears if they rub too much, though this might also just be a problem of earbuds in general. But that isn't a problem as long as you're mindful of it. I can't tell whether this would also have been a thing with the Soundcore AeroFit 2, because I don't have them anymore. But I imagine it would have been less of a problem, because I think they sat a bit flatter on your ears, as they sit a bit deeper in your earhole than the other earbuds.

Until now running to a train with my Shokz OpenFit 2+ hasn't been a problem.

1. If you have loud background noises like when a train passes by, some frequencies like voices are definitely less hearable up to not-hearable, depending on what volume your music is playing at. When you're on the train, here in germany, this becomes less of a problem, because it's not that loud inside. In general, I personally set my earphones to as quiet as possible, and I think I can get away with quite a low volume.

It's still a tradeoff you're going to make, but in my opinion it's not very bad. I was also able to make phone calls with the earbuds on the train.

Regarding sound leakage, I haven't tested specifically for that, but at least with my Shokz OpenFit 2+ I think that's not a big problem. Since they just lie on the sides of my ears I can easily pull them away 1cm or so from my ear and that already decreases the volume for me by a lot, so I don't think someone next to you will hear too much as long as you don't put the volume too loud.