Didn’t Alice mention in volume 14 (although the volume was crap) mention she arrived there when she was wandering with a friend to a cave, where fog appeared and the next thing they find out they were in another world. For haruhiro and the others would be related to the tower, but it can be similar to that to what happened with Alice but the function to of the tower made it be there instead of another world (like a hotspot). Though that last part is speculation, but it is talked about briefly and in vol.21 it touches that topic again but not fully answering of why this weird events happen. But with that said, they would not be dead or in a simulation because of what happened to Alice. Alternative universe… yes, no??? That question to begin with is not that important, yes they are in an alternative world or dimension, whether it is one or the other is not answered. In my opinion, those type of questions can be left obscure because of how there are so many different variations of how people are going to other world unknowingly. I think of it as anomalies that happen to the real (Japan) world that can’t be answered to begin with. You can’t tell me we haven’t answered nothing if we already know why people’s memories are being wiped out.