I think all five boys will be much happier without Amber around. She was right about about them swapping partners, but if she wasn´t comfortable about Sam being in gay relationship and having sex with Brett, she should have said something earlier on so that Sam´d known not to tell her details about it. It´s only natural to tell your best friend about your life, sometimes even about your sex life. At the end Amber seemed two-faced and manipulative and always wanting to have her own way. Not a nice quality in a friend.
About the engagement, I didn´t think it was too soon or forced, because I seem to remember them talking about getting married one day in earlier chapters. It seemed like an appropriate ending to their fall out and getting back together.
It´s been a lovely story