I just watched the 92Y interview with Peter, Sutton , Nico and Darren. Darren admitted they had no idea what they were doing from one season to the next. It shows. At the end of the last season they filmed Liza's cell phone ringing with a call from Charles. He went out of his way to say it was a BIG deal and we would find out what it was all about in the next season. But, the truth is, he only filmed that scene because Sutton Foster was uncomfortable with no hint of Charles and Liza in the final episode after a season of build up. And as we saw, they did not address it at all.
I think if he had his druthers, he would have dragged Charles and Liza out another season or two. He didn't realize how upset fans (and critics) were about dropping the story until after the finale aired. I am sure the 30% demo loss in this season's opener helped too. This writing willy nilly crap hurts the quality of the show. They should at least have an outline. Or something.
Or be deported. They could bring back the story of Liza needing to help confirm they are a real couple. But, she can't. Because of her guilt she spends time with Josh and the baby. Hence, more triangle.