I've actually been rewatching Heather's videos today and Carrie really does seem so worn out during it. Not everyone has the range for every part, even if they lower some of the notes for you. She couldn't go all the way up so she'd hit lower notes louder and it clearly took a toll. Early in the run she talks about "oh I got this cold" and it's like babe no you're overdoing it.
Also, I find it interesting how in the past she's referred to Addam's as her "uni experience" when in reality I think Heathers is too. The shit eating habits, all the coffee instead of breakfast, the goofing off, the sleeping in and then showing up to work early but tired, the constant sore throat, the flirting with other cast members shamelessly, the obsession with people screaming for her during Dead Girl walking, the collection of alcohol in her dressing room. It's all a little immature and attention seeking and self aggrandizing. The appropriate behavior for someone who is maybe 20 years old. Others in the cast are playing into it too obviously but I figure they go home and eat healthy, work out, and act like proper adults. I really do think she should have finished school, even if she didn't do uni. There's a time and a place for being silly and overdramatic and goofing off and I think if she'd done it at a younger age in a school type environment and gotten it out of her system she might be chiller and more mature now.