If the Spencer family behaved this way, did not support their daughter or sister when she expressed doubts about something as major as a marriage and a commitment before God, crown and country, then shame on them. Such a notion reflects extremely poorly on the Spencer family, more than it would ever have on Diana.
The one group of people everyone should be able to count on in this life is their family. If her family was so willing and eager to sell her out to the Windsors then they never truly loved her or cared about her, whatever and however much they cried over her and claimed to love her when she died.
They are a bunch of hypocrites if they heard Diana's doubts before the wedding and pushed her down the aisle to Charles anyways.
Yes, calling off the wedding days before to the future King of England would've resulted in tremendous media attention, critique, questioning and probing. It would've lasted a few weeks, even a few months. But then Charles would've gone on with his life. He would've come out a victim and the search for a new bride for the poor jilted-at-the-altar Charles would've gone on. In a few months the media would've forgotten about Diana. Maybe there would be some minor interest in her if she got married or something scandalous happened in her life, and some interest when Charles got married. But mostly she would've been left alone.
So for a year tops of some media critique and scrutiny, Diana could've had a lifetime of privacy and happiness, and hopefully a marriage free of doubts. Doesn't that sound better than the life Diana got by walking down the aisle to a loveless marriage?