Thanks for your questions.
Please see the attached slide for information.
A1: The 'Receiver Input Voltage Hysteresis' is used to prevent against unwanted glitches on the receiver output in the presence of differential noise on the receiver input.
The 'Receiver Input Voltage Hysteresis' is the difference between:
VTH for High to Low -AND- VTH for Low to High.
A2:The difference between VTH for High to Low and VTH for Low to High is typically 70mV.
The VTH is a minimum -200mV, and could be >
The VTH is a maximum +200mV and could be <
The Receiver Hysteresis is the difference between these, and not necessarily centered around 0V.
A3: In the diagram you attached: you asked - 'The output is not set to low because ^VTH is less than 70mV' (Is my understanding correct?)
-> In the case that you have described, yes the output will not be a '0' Low as the signal falls within the Receiver Input Voltage Hysteresis region.