Weren't the crew already doomed before WAU

[Abion47]

When Simon-2 reactivated the power plants at Upsilon, that's what seemed to activate Catherine, since the first communication with her happens immediately following turning on the turbines. So in that sense, had Simon never been in Pathos-II, and assuming that the power plant hadn't come back into operation by some other means, the mockingbird known as Catherine would never have existed at all.

Had the other scenario happened, it would depend on when Simon would have died. If he died while she was plugged in somewhere, then I imagine that, yes, Catherine would have slowly gone insane. Not the violent kind of insane, though, but the kind of insane where you are left completely alone for days, weeks, months, or years with nothing but your thoughts. You slowly start to just withdraw from the world and enter a sort of trance, a waking sleep in which nothing seems real and time stands still. She would probably try to keep doing something for a while, of course, like maybe trying to transfer her consciousness into a robot with some form of mobility, but depending on where she gets trapped, it's unlikely she would find host body capable of making the trek to Phi and launching the ARK, if she found a body at all. Ultimately, though, her fate would be similar to that of Simon-3's - trapped without a purpose in a world of darkness and fear, just waiting for fate to decide to finally pull the plug and end it all.

On the other hand, if Simon had died during the times where Catherine wasn't plugged in, then Catherine wouldn't even know about it. During those moments, time truly does stand still for her. To be honest, I consider this to be the scarier possibility, to be trapped in a temporal limbo, waiting for someone to come along and revive me back into the world. But that moment could potentially never come, and she would just forever be trapped in a state that is less than sleep, less than comatose, but simultaneously existing and not existing. In that state, she still exists conceptually, but in terms of her own reality, she may as well have never even been born.