The Cube definitely has appeal to people of all ages, just based on the people who've come up to me asking for more info when I've been driving mine around. I'm searching for three or four baby boomers at the moment.
To answer your questions:
Q1. Probably, although the Cube is a fair chunk smaller on the outside and has a much smaller engine. The Cube also has a 7 seat option that the Rukus doesn't.
Q2. Younger buyers will probably mod them with aftermarket gear, the factory individualising stuff is best left to people who buy Kia Souls. Older people won't really car that much.
Q3 & Q4. I doubt you'd see a Cube dip under about $7-8K unless it had really high mileage. The Cube will depreciate better than a Rukus purely because it doesn't have as far to fall. I think if the Rukus had been sold here from 2005 or whatever, it might've been a different story, as there would be used examples of both on the market to get a better comparison. It's a bit of an apples and oranges comparison, but even still, a used car will always depreciate better than a new one.