(lets forget about X11 breaking all this assumptions
Indeed.
While experimenting with module loading disabling, I experienced that broken X can block switching to virtual console. Needless to say (for other readers), if X can do, also malware could do. “SysRq + r” can take away control from X. After that, switching to another virtual console was possible.
Yes, X (or other process with access to input device) can grab it for exclusive access, disabling Alt+Ctrl+F1 or similar combos. This still is independent of what is happening on other terminals. Especially, input devices grabbed in this mode are handled by X server (or other process that grabbed them). As long as X server doesn’t have access to other terminals, it still can’t influence them.