I had to go back and listen to the studio version and it does sound like Carrie sings at least two layers in the chorus: a lower one that is the main vocal and then another higher harmony, which is the one she sang live. So I don't think she's singing anything differently than from the original, it just came out harsh. The last chorus where she sang it lower was somewhat better, but still not the best vocal (except the verses). Glad Carrie had the opportunity to sing on a new stage and with David though. Looked like they had fun.
Also, limiting speech on a public forum to only "positive" discussion and labelling people as "fake fans" when they voice their thoughts and call things out sounds cult-like, tbh.
I actually just realized this today, too. I had never listened to the song with headphones on before, but I did notice the top harmony in the background for the first time earlier today.
My theory is that she was either having trouble hearing herself during the performance, or she assumed that whatever initial problem they were having with the mics was still ongoing, which resulted in her overpushing the vocal to compensate for the perceived sound issues. That, combined with the adrenaline rush of actually performing in front of a live audience again for first time in almost 2 years, just made it sound all the more strident.
Also, thank you - I agree. Most of us are all grown adults at this poin;. I'm going to be 30 in less than 3 years from now. The "real vs fake fan" accusations were a little more acceptable when we were 12-17. After that, it gradually becomes less and less cute and more concerning and pathetic.