4. You'll get higher noise levels on the other side of the speaker(s). With the speakers outputting sound that is opposite in phase to the noise towards the front, behind the speaker you'll get noise that is IN-phase with the noise, making that side louder. If you have the speaker mounted in the case, then you'll likely have more noise inside the case, which you'll have to deal with.
It's far from that simple.
The setup will create an infinite number of sound minima and maxima in space around it, so by moving around you will find some quiet positions and some that are more noisy than without the cancellation.
It only works well with headphones, because the headphones only need to cancel noise at one point in space - the ear canal - which is not moving relative to the headphones and which is very near the cancellation device.
Let's do a very simplified example:
Assume that noise is not attenuated by distance from source. Assume that we have 2 noise sources in free space, producing the same sine wave at frequency f at the same intensity and in the same phase. Assume the distance of the listener from source 1 is d1 and source 2 is d2.
Now the other source is d=(d1-d2) further away from (or nearer to) the listener than the other.
This difference in distance creates a 2*pi*f*d/speed of sound difference in phase for the 2 signals in the listeners position. This creates interference. The intensity of the sum signal at the listeners position is 1+cos(phase angle), which is a value between 0 and 2. So depending on the position of the listener in space, these 2 noise sources either totally cancel each other or add up perfectly or something in between.
Now take away the simplifications and you notice that it becomes totally impossible to do. For example, the location of the minima and the maxima depend on the frequency of the sound, and it will be very hard to do it for all frequencies at the same time. But it also depends on the attenuation and echoes (by frequency) created by the listening space.