^Yes, but Marcelo was still in the picture by that time. He married Tarja in 2001, but she knew him and was dating him long before that. Marcelo credits himself (most undeservingly, I think) with "saving" the band because supposedly he was able to get them to reconcile. This I doubt, because Tarja said that Tuomas just decided he wanted to keep going and not throw away his success, not throw away all his hard work - which makes more sense, really.
We don't know the exact circumstances or what all the issues were - all we know is that they were, obviously, resolved. Marcelo also claims they were happy while touring for CC and things seemed well. This I also doubt, partly because it's Marcelo talking and we all know he isn't exactly the Prince of Honesty and Forthrightness, and partly because from everything I've heard and read, the guys and Tarja were never really friends in the true sense of the word.
So, you asked, why did it end the way it did with Tarja, when they don't spend all their time with Anette either? I think it's because they chose Anette when they didn't have to. Nobody was making them, nobody was advising them. They chose her because of her voice, of course, but also because they really liked her as a person. There's a key difference. You can say they 'chose' Tarja, but in my view, not really...they chose her because she was there. More of a matter of convenience, and being young, inexperienced, and nobodies in the music industry at that time. She was someone they knew, but they weren't friends. They didn't know any singers very well, and Tarja had a family connection to Tuomas - I believe Tuomas' mother cared for Tarja's little brother or something. So they had a connection, but it wasn't a very personal one. They needed a singer, she was there and she agreed to do it.
Of course, it goes without saying they liked her voice or they wouldn't have kept her. But they didn't realize their project would become so big. If they had, perhaps they would have chosen someone they were more compatible with, someone they really knew. But they didn't. They never thought they'd be stuck on a tour bus with this girl for months at a time. Tarja didn't like them very much either in the beginning, from the sounds of it. They had to learn to like each other, because they had unwittingly become the most famous band in Finland without even trying, and all of them wanted to continue...but the difference is that the guys were already friends. Tuomas and Emppu were childhood friends. Jukka was a childhood friend of Emppu's. And, fast forward ahead a bit, Marco was already in so many metal projects, he knew what to expect from a tour, and he had already toured and been around a bunch of drunk guys. Tarja had none of these bonds, and none of these experiences. So of course it was harder for her.
Back to Anette - Anette had a good idea of what she was getting into. She knew the band was famous, she knew there were huge expectations, and she knew she'd be spending months at a time on a tour bus with four guys. Tarja knew none of this at the beginning, she simply couldn't have. And the guys CHOSE Anette, out of a virtually endless list of other options. The fact that they chose her counts for a lot. And Anette keeps in touch with them when she is in Sweden, she would talk about texting Tuomas or Marco or Emppu all the time in between DPP and iRum. It seems like they genuinely like each other and want to get along. And, everybody in NW is an adult now. The guys are all over 30, Anette's 40, Marco's 45. They're all getting a little too old for the same antics they had back then, and they all know it. Older and wiser, as they say.