How did the cessation of crossing-over contribute to the disparity between the X and Y chromosomes? ...

Answer to q. 1

In most chromosomes, crossing-over is a way to make sure that subtle changes are regularly
exchanged and that major differences do not accumulate. In the X and Y chromosomes, the slow
accumulation of a region in which crossing-over did not occur prevented genetic exchange between the
two and resulted in each evolving differently. It is very similar to taking a population and separating it
- each will accumulate its own unique mutations and these will slowly create a significant difference
between the two.

Answer to q. 2

If the autosomal dominant trait is either lethal before the individual can successfully reproduce or has
enough disturbance on phenotype to prohibit reproduction, the afflicted individual will not reproduce
and the trait cannot be passed on.

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It really depends on the course, never give up