I much prefer land value taxes (and similar taxes on non capital wealth like jewellery) and leisure taxes (ideally taxing people for every hour they don't work). Of course these are difficult to administer in practice, but British business rates and US overtime tax discounts effectively approximate this.
I think something more like "investment income should be taxed at a higher rate than income earned through labor," accomplishes similar goals but is more intuitive and less problematic.
The time we actually know what money they would make, making it concrete for tax, is when they sell. And we already do this.
No it's not. Most people don't pay capital gains because things like houses don't have capital gains applied. I.e. the rules are set up so that most people avoid capital gains tax when they gain capital. Everyone pays income tax with the same rules, and some people pay capital gains tax as well if their risks pay off. We tax the profits and let investors absorb the losses.