I don’t know enough to elaborate on String Theory- I just think it’s an interesting thing to think about. There have been critiques of modern physics on this forum here and probably elsewhere on the forums if you search for them (if interested). I wouldn’t say that it follows that because something cannot be observed that that thing isn’t scientific (space and time are assumed to exist for every object one even sees–yet they play critical roles in scientific analysis, especially physics and chemistry).
Are there physicists who deny that something preceded the Big Bang? Perhaps you could cite some?
I haven’t read the Objectivist links, so I’ll have to respond later.