The reason for the white images is because when outside, there's LOADS of light available. So when you take a long exposure, it's exposing the sensor to ALL that light for a long amount time.
A nice, properly exposed image on a sunny day will probably have a shutter speed of about 1/250th of a second. That means that even when the sensor is exposed for just 250th of a second, it's able to get a nicely exposed image.
So think about how much light is hitting the sensor when you use slow shutter speeds like 1/15th or slower in bright light. All you're gonna end up with is a washed out image.
The only way to have slow shutter speeds in bright light, is to close the aperture of the lens (use a high "f" number), or to use a filter like a neutral density filter, which acts like sunglasses for the lens and essentially cuts down the amount of light that hits the sensor. So using an ND filter, it takes a longer shutter speed to achieve the same results as if you were to just use "normal" settings, but it means that any movement during that longer exposure time is also captured, and this is how people get those motion blur/smooth flowing water effects.
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As for the S setting automatically setting your aperture, that's normal. Say you want a fast shutter speed of like 1/250, you set the shutter speed yourself since you're in shutter priority mode, but the camera needs to compensate by adjusting the other settings itself. So if the sensor is only going to be exposed for 250th of a second, it's going to need a lot of light to come in, and so the camera will set the aperture wide.
Likewise if you have a slow shutter speed like 2 seconds, then there's going to be a lot of light coming in during those 2 seconds, so the camera needs to compensate by closing the aperture to restrict the amount of light that comes in.
However unless you have the camera set to some kind of automatic ISO, then it shouldn't be changing the ISO setting. So I would double check your manual and/or settings to see if an automatic ISO is selected.
Also bear in mind that the P100 is a bridge camera. The sensor is tiny compared to DSLRs, and the lens doesn't have the versatility of DSLR lenses, so don't expect the exact same settings one might use for a DSLR to work the same with your bridge camera.
