I think it was pretty safe to say that Dana wouldn't have been in GB3. All the old news never mentioned she was going to be in it. Sigourney Weaver never seemed like she was approached about it. If so it was speculated that she'd have a cameo to appear so that the film showed she was married to Peter and moving on with their lives. If Bill Murray had done only a cameo, it was supposedly Peter wishing his buddies the best of luck from a phone booth. So even again Dana may not have been in it at all.
No doubt Ackroyd and Ramis coming to their senses that it would have limited telling a new story if you completely stuck with the same characters (especially if Dana's problem was the focus again).
The same philosophy applies right now to Terminator 3. They recasted John Connor to have him as a man (maybe due to Eddie Furlong's drug/drinking habits and he don't look like a man) and Sarah Connor is dead by then in the story's present (the future). They wanted to not have been limited in storytelling if all the same characters were in it. The chance to break out and bring things with a fresh approach was decided. But I also wonder how good T3 will be without James Cameron. Can't be half bad if their: spending $170 mil, Arnold liked the story, most of the same producers are still there, Stan Winston on make-up and models, ILM on the SFX.
I agree Dana didn't need to be in GB2.