Yes, the chip is detected and I'm able to read the chip just fine. Would this indicate everything is hooked up properly? Or could missing the /HOLD /WP /RESET cause write issues, but not read issues? Knowing that would really help.
In general, yes, the behavior for disconnected inputs is undefined. Some chips (like the Winbond W25Q-series) have internal pull resistors that allow for pins to be left disconnected (the datasheet explicitly mentions those). I don't see that in the Macronix datasheet though.