For clarity, in order to be a "stud", the framing member has to be running vertically. When it is running horizontally, to support the floor, it would be called a "joist".
U-boats are indeed designed to isolate the floor from the structure below. There are many valid applications, usually when you are "floating" a floor to isolate vibrations and sounds so that they don't travel out of the room and into the building structure, and visa versa. This is normally done for the entire floor, for the same reason you would build independent walls or use chips and channel. I don’t see a lot of benefit from isolating the riser from the floor below. Vibrations will transmit through the floor in all the areas that are not covered by the riser, and you are deliberately letting sound into the riser to the floor under it.
As for venting, vents into bass traps that are near corners of the room are more effective. Vents that are not at corners should still provide benefit, just not as much. Areas that are not vented won’t add benefit. This isn’t a black and white issue, and using a riser as a bass trap is just a way to get something “for free” from a structure that you are already building.
You might be interested in this thread that I just commented on. https://www.avsforum.com/forum/19-de...does-work.html
I can’t speak directly to U-boats damaging a floor or not. It all depends on what kind of floor will be under the riser. Carpet would likely get crushed and may not spring back years later, no matter what you put on it. Areas that are not covered may fade more than areas that are covered by the riser. I can say with certainty that rubber can discolor some types of flooring. I once had a cart with rubber wheels on a vinyl floor and the coloring in the wheels leeched into the vinyl flooring, permanently staining it.
More layers of flooring, particularly if you use Green Glue between the layers. Is less likely to resonate and cause discolorations in the sound. If you bang on the top, you don’t want the riser to sound like a drum. A dull muffled “thud” is much better.
If your existing floor is stable, you really don’t need to do anything to it, other than perhaps putting down something to provide a slight cushion and to protect the finished floor underneath if you ever plan to expose it again.