I have a question. Was it established whether or not the guy watching Clair in the courtroom (with Cliff) was Mr. Lucas, or a random observer? Was the actor used later for the actual role of Mr. Lucas, in a similar way that Joseph C. Phillips started out as Sondra's boyfriend, then became Denise's husband?

Because it seems that the guy ("Mr. Lucas") didn't realize that Cliff and Clair were married. When Cliff said to Claire "maybe we can get married" then "Mr. Lucas" looked flabbergasted, as if to say WTF.

Then, in The Juicer episode, it seems as if Mr. Lucas is meeting Claire for the first time.

Can anyone clear this up for me?

The actor you are referring to is David Langston Smyrl. He appeared in 5 episodes of The Cosby Show. His first appearance was in Clair's Case in Season 1. He didn't have a name there. His IMDB entry for that episode just says Man in Court. His second appearance was in Mr. Quiet, also in Season 1, as Chester. His last three appearances were all as Mr. Lucas, twice in Season 2 episodes The Juicer and Denise Drives, and lastly in a late Season 3 episode, Andalusian Flu.

http://www.imdb.com/name/nm0810759/?ref_=tt_cl_t10