At present, the “none” option is ranked higher than any of the colours. This makes no sense: if you are labelling something, presumably it has some greater significance. I suggest “none” should rank last when sorting by label.
Sorting by labels uses the name of the labels, just like in the Finder. But it’s possible to use ascendant/descendant sorting.
Yes, but if you do that – reverse the sort order – while you get the unlabelled items at the bottom, within each label items are sorted in reverse alphabetical order.