As you say, charges being dropped are down to the chances of securing a conviction, which rest on the willingness of the victim to testify. The major difference between the Arsenal player, who has previous allegations against him, and Greenwood is the victim's video, which has made up the minds of most who saw it or are aware of it. Greenwood has already been tried and convicted in the court of public opinion, which is going to bend the willingness of the club to retain him.
It's not right in the slightest, but Gooners are able to turn a blind eye because it's his word against hers/theirs. That's not possible with Greenwood, so I think the backlash would be enormous if a domestic club sign Greenwood.