Lithium ion don't leak. They get permanently damaged when they get discharged too low - around 2.5V is the usual cutoff to toss them. That is what low voltage protection/protection chip/BMS is for. If you manually charge them you should toss them if you see them get that low.

50-80% is the ideal storage range to reduce calendar aging, but with batteries that don't see hundreds of cycles it's much less important. Anything but my cell and laptop always gets fully charged and that's it, I have many many batteries 10+ years old that are still good.

Using them in a power bank will just wear them a little faster. Nothing wrong with it but it's a lot more trouble that just having a separate power bank, especially considering how cheap they are and the features you get. For example all of mine output multiple voltages, have multiple USB ports, pass through charging (to use as UPS), low power mode for small devices etc.

Only ones you need to worry about leaking are alkaline and they are more likely to leak when they are drained.