Any time you rely on an automated metering-based system to handle exposure for you, you're going to get inconsistent results shot-to-shot, because the scene change and the exposure changes. In addition, we don't know how your camera is set, and flash behaves differently depending on the camera mode as well.
In M mode, the assumption is you know what you're doing and the camera won't interfere with how the flash is set.
In Av and Tv modes, however, the assumption is that you want flash for fill, and that you're balancing the ambient (non-flash) light against the flash so that the majority of the light will come from the ambient. You can nix this behavior in Av mode by messing with the custom settings and nailing down a faster shutter speed in Av mode. But in general, exposure settings will be very close to those you'd use without any flash at all, and only a little bit of flash illumination. This could be why your first image seems underexposed.
In the P mode, the flash will behave more like you probably expect it to from experience with built-in flashes on P&S cameras. In good light, the assumption is that you want the flash for fill (i.e., to "fill in the shadows"), but that in low light, you want to use the flash as your main source of illumination. In this case, you tend to get an image much more like your second one--where the majority of the light in the scene is coming from the flash and not so much from the ambient.
Every flash image is basically the sum of two exposures: the ambient light (all the light that isn't from the flash), which is controlled by iso, aperture, and shutter speed; and then all the light from the flash, which is controlled by iso, aperture, flash power output, and flash-to-subject distance. How you choose to balance the flash against the ambient will determine the "look" of your image.
As for your white balance being all over the place--yes, this happens. Flash tends to be a different color from ambient lighting. Typically cooler/bluer. With mixed color light sources, it's difficult to white balance easily and simply. The method to get around this is typically to "gel" the flash--that is to put a colored filter over the head of the flash, so that its light more closely matches the color of the ambient lighting.
See also: How can I get started with a first flash gun?
And the neilvn.com (Neil van Niekerk's website) "Tangents" blog, most notably the "Flash Photography Techniques" section.