To answer some of your questions you have to see it in context.
Up until Nodoka's arrival Ukyou and Shampoo were with Ranma like in the Canon just like the others girls.
After the introduction of Nodoka things changed and she was the main reason that all the girls changed. The changes were subtle and gradual until it cumulated into the series of events that happened to Ranma. They played their parts because they wanted their honor intact. Ranma, on the other hand, had no clue that this was going to happen. She has her own demons that will be explained later. It does not make it any less heinous as what she did to her own son, but does shed light on why she did what she did.
As to why the girls had changed.
Let s face it. It s easier to tear down a person than to help them. An example is that high school gossips can destroy a person s reputation even if they knew them. If the gossiper is credible to them then they will doubt the person based on someone else s say so, but they will not go to the source which is the person because the person might be lying. Or they follow whoever leads. Nodoka will fit the bill perfectly. Nodoka was the mother of Ranma, so she should know, right? She was a mother figure that the Tendos girls, Ukyou, Shampoo, and Kodatchi (on par with her) were looking for. She has a high status family, strong willed, and was what was consider a Lady (has the Honor Sword). In short they looked up to her. When she started to subtly slander Ranma it was taken as a cue for them to do the same.
The section of the meeting with the gods is just that...a meeting to tell them the situation. Just like children the other deities wanted amusement since they will live practically forever. Kami does permit them to make few changes, but it must be SUBTLE with no direct interference in the lives of mortals. He s giving them the freedom to make mistakes, but if it gets too much then He will take matters into His own hands. He has stopped them, but what happens in the aftermath will be totally up to Ranma. There will be some interaction with the deities, but that's for later.
It might seem cruel, but Kami will not change time just for one person. As many people would say, If life throws you lemons then squirt it in the eyes of your enemies and beat the crap out of them. Shit does happen so Ranma will have deal with the dirty laundry.