Those who use the Longo editions often play different notes because Longo "corrected" much in the Sonatas. Scarlatti made unconventional choices which where assumed to be mistakes. Longo also added voices because Scarlatti often drops his voices and lets them return later. His music really wasn't understood by some scholars. But there are later editions that are consistent with the original manuscripts, so you will hear different versions.
It seems my Henle and Fadini editions have different measure numbering, so I am not sure which measure you mean?
When it comes to "filling" harmonies with "missing" voices, I would not, since it is quite clearly not what Scarlatti intented.