Is it normal to use a softbox for a back light?
It can be sometimes. With two softbox lights, one can very nicely, evenly light a background show here. Note in the video that the subject is much farther than 2ft from the background.
Wouldn't it leak light onto the wall behind the subject?
Yes, it would, but not because it's a softbox. Softboxes only cause less harsher shadows. The reason it would spill light all over your scene is because the light inside of the software box is not directional at all, but rather an open-face bulb.
Maybe those concerns can be addressed with the use of grids?
The purpose of a grid is to shape light and contain it. So, with that in mid, a fabric grid would help reduce spill onto your subject and wherever else you don't want it.
Would a softbox like the one above work if hung directly above the subject who has their back one or two feet from the wall?
It really depends on what kind of look you're going for. What color are your walls? If they're white, the light will likely reflect off of the wall and bounce back onto your subject creating a rim light around them and illuminating the back wall. It won't illuminate it evenly (a single softbox light), however, and will likely have a gradient from light to dark, top to bottom.
If a reflector light is better, why do most of the kits include light boxes on the booms instead of reflector lights?
Nothing is better, per say. Light is highly dependent upon the situation. You as the videographer need to figure out which lights are best for your situation.
I would like to take a moment to say that when it comes to light, the whole motto "buy once, cry once" i.e. get good quality lights, rings true. The kit you linked, while I've never had experience with, will likely not last you very long and be a poor investment. You should consider purchasing used, well known name brands, or rent until you can afford real lights (if budget is a concern). Again, I don't know what kind of production you're running, but this is the advice that I've come across time and time again. A good starter set for interview style set ups, is this guy. Additionally, tungsten lights are cheap and render skin tones very nicely. Some brands to find used Tungsten lights are Arri and Mole-Richardson. Try to stay away from off-brand LED lights.
This is only scratching the surface.