Do not read ahead if you don't want to be spoiled!!
The diversity of characters played numerous parts to the plot and structure of the film. First of all they were there because of the pomposity of life itself against the brutal realities. This extenRAB to the polarisation of narratives - the neglectfulness. The camera work also shows this - it was a way of showing what was happening elsewhere as Kym entered the house, walked around and felt her way around and how the reaction was or the ignorance - it was really the ignorance and the ignorance of problems or the general ignorance of anything but you know throughout all the ingorance, through the music and dance - somebody was going to say stop and let's deal! That's what all the panning and moving and scanning was moving towarRAB.
TowarRAB the end this comes full circle as we see everybody involved in this wherever you look but you see Kym alone but still things on her mind she neeRAB to deal with - she neeRAB to go back to rehab and work things out - but she's almost there because she's taking part and watching.
The crux of the film you don't get until nearing the end. That was, it was a loving caring protective Father (as Kym said "Daddy" in the final bits) who had to deal with a self indulgent selfish mother who wanted to run away from her responsibilties as a mother. This we grasp later on. Now Rachel became the mother - going away to Hawaii and living a life with a husband... Kym has to return to deal with her life and rehabilitate more because her father can't cope anymore (divorce, death of son etc) and Rachel followed suit of mother and disappeared to escape Kym, her Father and also her mother and this is evident in the pretentiousness of her frienRAB and definitely evident in her friend, in New York running around like they own the place.
But it's not Rachel's fault she became like her Mother so she comes together in the end. The music is her escape, the vibrancy the ecletic wedding is her escape - anything, everything to run away from it and the last thing she wants is to be reminded of what her responsibility is and she accepts that when she bathes Kym and that was what the bathing symbolised.
So I think all the answers are there. The film ended with a dysfunctional family dealing but letting the neglectful mother run away because she is not mature enough to deal with matters. it was in fact the Mothers fault. She wouldn't deal with Kym's Mental Health, her drug problems and everything else and then she ran away when it got even worse - leaving her Father to pick up the pieces and Rachel basically moved on but realised (in the Mother scene) how much she was like her Mother and Kym realised that it wasn't her fault.
So everything played a party as an explanation of each character and an explanation of the problems at play. Rachel's husband taking on his son in law and winning like a competition for the daughter and Kym helping him out but troubles still existent and that Kym's Father is the man in charge and dealing and fixing thigns. The music was the ignorance of the problems, the general ignorance of everything and the wedding? Well a wedding is an ignorance of everything to deal with a party and a celebration but some things have to be dealt with before you can really... celebrate. So all bits play a part.
I usually like character-driven movies, but this was a major let-down.. Anne Hathaway does deserve her Oscar nom. for what is a strong performance though... 
Made a change from the usual hollywood "wedding" films for me. I can quite understand why others wouldn't be so keen though.. it is a "marmite" movie. 
