I think a lot of the deviations come from wanting a dramatic scene with all players present. So we get the evening party at Pemberley instead of the morning call. While I miss the morning call (before & after Darcy as you described it!), I do like having the Gardiners both present so we see the contrast between them and the Hursts, for example. Mr Gardiner looks so much the gentleman, for example, and there's drowsy Mr Hurst.

This scene in P&P2 allows us to see Darcy looking at Elizabeth as she plays & sings, as well as letting Darcy look his gratitude & admiration when Elizabeth goes to Georgiana's rescue. And this is the scene where we get a glimpse of Elizabeth's changing feelings without using a voice-over or diary or letter to describe it.