Knockout and knockdown of gene
This is a fascinating set of questions.  The only one I can comment on is: is KD qualitatively similar to KO?

My answer is: sometimes yes, in general no.  To understand why, I like to think of a hypothetical plot of "phenotype of interest" on the y axis, and "expression of gene" on the x axis.  You can imagine that this plot could be monotonic, i.e. that the phenotype continuously increases or decreases as expression increases.  Or it is also possible that this plot could have one or more minima or maxima in it.  One example of KD giving dissimilar results from KO is for essential genes.  If they are knocked out, they of course kill the organism.  However, there may be some levels of essential gene knockdown for which the phenotype is actually increased relative to wild-type!

I will selfishly share one of my articles where I observed something like this.  We found that ADH1 knockdown improves butanol tolerance in yeast, but that ADH1 knockout was highly detrimental to butanol tolerance.  http://dx.doi.org/10.1007/s10295-015-1713-7