Joan Crawford would have been a disaster in the role of Liz. Joan Bennett had a strong background in theatre, which was why she was so perfect in the role. Granted, Joan Bennett occasionally forgot her lines, but she maintained her regal composure to the end onscreen.
This is just personal, but I always felt that our Joan exuded a certain warmth, even when she was laying down the law in what appeared to be an unreasonable fashion to Miss Winters (in the early weeks). I've never felt warmth from Joan C (although I admit I enjoy her film performances enormously).
I wonder if the "other actress" who "talked to" Joan C on the set of the Lucy show was Viv Vance. I could see Viv doing that in a sensible, yet sensitive way, that would have gotten the message home without making Joan get her dander up.
G.