1. Vertical and horizontal biamping are two ways of connecting the amps but both are passive biamping if they are not implemented with an external active network before the amps and with the internal crossover removed/bypassed.
2. It depends on the music, of course, and that division is certainly possible. However, the power distribution is much more skewed to the bass in most cases. In addition, not everyone's crossover fits the bill.
3. However, it is still hard to see how "beneficial" it can be. Even if it was 50:50 power distribution at the user's low/mid crossover, the absolute maximum effective power increase one could realize would be 3db, the same as you would achieve with a single amp of double your present power. I see no other advantage with a passive arrangement.
4. OTOH, unless you miswire it, there is no downside to biamping and those who are intrigued by it should simply try it for themselves.