I think that the WTS would distinguish between actual, physical adultery and adultery "in the heart." While both would be condemned as wrong, and while some types of pornography can even be grounds for disfellowshipping, I think they would contend that if no "actual" (i.e., physical) adultery had occurred, there would be no grounds for divorce.
As far as needing two witnesses for adultery, that would only apply if the person being accused denied that he/she had committed adultery. However, remember that judicial committees will accept as proof of immorality two witnesses to the fact that the accused "stayed all night in the same house with a person of the opposite sex (or a known homosexual) under improper circumstances." (Shepherd the Flock of God, p. 129, emphasis theirs). So if two elders hide in the bushes outside a woman's house and see a man to whom she is not married enter in the evening and leave in the morning, that may be all they need, even if he was just there to repair her plumbing.