I suspect that few remember that Mork and Mindy was a spin-off of Happy Days ...
And even fewer are aware that Mork was just supposed to be a dream of Fonzie's. When Robin Williams's performance became an instant hit and Garry Marshall decided to do a Mork spinoff, they re-shot the ending of the Happy Days Mork-dream episode to retcon it into something that actually happened but that Fonzie was made to believe was a dream, and reran that episode with the new ending just before Mork & Mindy premiered. Since the revised version of the episode is the one that went into syndication, it's largely forgotten that Mork originally wasn't supposed to be "real" at all.
I suspect the WGA decision had a big basis on who's idea it was in the first place, which favors Roddenberry, along with just how much was left of Black's version after GR was through with the rewrite. This requires a direct comparison with whatever materials both men provide to support their case, i.e, Black submits his version, Roddenberry submits his, and the arbitration board compares the two.
Who gets screen credit determines who gets paid, so these sorts of things are not something the Guild takes lightly.
Right. If the WGA gave Roddenberry sole credit, that means that, in their judgment, he was responsible for the majority of the script's content.