I appreciate the work you’ve put into 2.5.3 and hesitate to quibble but smart quotes still does not correctly convert English single quote marks. This is slightly difficult to explain in words without using the symbols themselves but here goes.
The double quotes conversion in WordService is correct, that is, when the quotation mark appears after a space the double curly quote is left-facing. After a space the single curly quote should also be left-facing instead of, as is the case now, facing in the same direction as the closing quote. In other words, when you use curly quotes in English, the opening quote and the closing quote, whether single or double, should face in opposite directions.
I know it sounds like a trivial issue but it really does create a lot of inconvenience if having smartened the quotes using the service, you then have to search and replace to fix the opening single quotes throughout the text.