I can understand the frustration.
Since she is on campus, and literally every Chinese student has 1000+ contacts on Wechat...shouldn't that be her first port of call?
Accounts can and do get blocked when changing SIM, but only when trying to re-enter credentials and it can't be verified. I swap out my SIM cards every month or so when I'm travelling and not had issues. I can't comment on the unusual greetings issue...
Sorry for coming across as obstructive, but especially for the next couple of weeks, need to be a little vigilant on social media...
Finally, although it may seem like the end of the world in China not to have Wechat...it's not. I still have people using old fashioned text messages to me from time to time...it's super cheap and reasonably effective. Yes, it lacks the 'cool' features and genuine strengths of wechat. But for just keeping in touch for a few days until this gets sorted, might be the way to go. Would also recommend your daughter asking a Chinese friend to talk with customer support (in Chinese)...
And really finally, can always set up a new username...friends who got locked out when moving back overseas did this, and it's a pain when you have years of wechat conversations that you've lost, but that's not the case here.
All the best to your daughter!
tb