If Shepard chooses synthesis ending (green one) - what actually happens to Husks? Are they dead or they become sane?
According to Mac Walters, the creative director for the Mass Effect franchise from Bioware, they are not sentient anymore. He mentioned that in a video, in collaboration with Ars Technica, where he responded to unsolved mysteries of the Mass Effect Universe.
Specifically, the question he answered, which is related to yours, was:
Do Reaper husks retain any of the personality of the converted individual, even if they have no control over their own actions?
And Mac's answer was:
The answer to that is no. Reaper Husks are husks. You must kill them, do not hesitate if you see one, destroy [them].
This can be seen in the following video (see 1:01):