How does the length of a wave train and its incoherence affect the spread in wavelength? The following text is from Concepts of Physics by Dr. H.C.Verma, from the chapter "Light Waves", page 370, topic "17.9 Coherent and Incoherent Sources":

Because of the incoherent nature of the basic process of light emission in ordinary sources, these sources cannot emit highly monochromatic light. A strictly monochromatic light, having well-defined single frequency or wavelength, must be a sine wave which has an infinite expansion. A wave train of finite length may be described by the superposition of a number of sine waves of different wavelengths. [...] Shorter the length of wave train, larger is the spread in wavelength.

I have resolved my questions along with the associated text:
Question 1:

Because of the incoherent nature of the basic process of light emission in ordinary sources, these sources cannot emit highly monochromatic light.

How does the incoherent nature of the light source affect the spread in wavelength (inability to produce highly monochromatic light)? I think incoherence/coherence is just related to the phase difference and not wavelength. So this statement is confusing for me.
Question 2:

A wave train of finite length may be described by the superposition of a number of sine waves of different wavelengths.

Why is it not possible to describe a wave train of finite length by a single wave instead of superposition of multiple waves?
Question 3:

Shorter the length of wave train, larger is the spread in wavelength.

In another paragraph, the author says lasers emit very long wave trains of the order of several hundred metres and this is why the spread in wavelength is small. Why should the length of the wave train affect the spread in wavelength? Aren't the length of the train and its wavelength independent of each other?  

Related: Why, in order to obtain distinct interference, is a small distance between the two waves essential? (based on text from the same topic)
 A: 
How does the incoherent nature of the light source affect the spread in wavelength (inability to produce highly monochromatic light)?

Coherence means constant phase difference between different parts of a wave. If a wave is incoherent, it can be described as a superposition of short wave packets, each with a random shift of phase and position of the amplitude peak with respect to the others. But since these packets are shorter than the whole wave emitted, this broadens the spectrum.

Why is it not possible to describe a wave train of finite length by a single wave instead of superposition of multiple waves?

Note that the author says "a number of sine(!) waves", not simply waves. And sine waves have, by definition, infinite extent. So to be able to get a shorter wave packet, you need to have a superposition of these sines.

Aren't the length of the train and its wavelength independent of each other?

There's a definite uncertainty relation between frequency and time (and similarly, length of wave packet and wavenumber (which is directly related to wavelength)), known as Gabor limit, so they are not independent.
