OP I think that you did the right thing by not going on the holiday - you were proven 100% right that there was something “off” by their behaviour over the past year.
You are not being consistent with your story around the “test”’- was it a real situation or not?
Either way you did diplomatically bring it up with one friend - so you did talk and confront - but this was futile as she wasn’t helpful. I am not sure how realistic it would be to bring it up with the whole group face to face at once.
Unless there is one “Queen Bee” then who are “the group” and what did they “all do” - is it not a series of reactions by individuals.
Clearly the girl who got back in touch told them that you had raised concerns with her - but they decided not to address those but sent you to Coventry instead. Sounds like looks of communication failures and “off” dynamics in the group......which you had sensed already.
With groups like this once the first scapegoat has departed they start on another one. This is the texter.
She is either looking for gossip on you to update the others - but has already told you that she won’t be informing you what the issue was - so its a one way street. Don’t go up it.
Or they have ousted her now - and she needs friend - hence all the flattery.
Tough shit really - she didn’t have your back - and actively betrayed you. I wouldn’t accommodate her - just secretly enjoy the fact that you were right all along, she behaved badly and has now been stung.
Leave her to it - her behaviour was unforgivable. Invest in new friendships.