Terminating resistor considerations for half-duplex RS-485 over 80-200 m cable at 115 kbps
All your questions seem to be about how to calculate if termination should be used or not. This is an XY problem. Simply always terminate it so there is no need to worry if it works without termination.

Yes, standard transmission line rule is to terminate with cable impedance. On the other hand, some systems that use RS485 always specify to always use a certain termination regardless of cable impedance. This is simply because default infrastructure (devices and termination plugs) come with default termination value and it would be unrealistic to assume that everyone knows the cable impedance and has matching termination resistances in their pockets when installing something.

Usually you buy cable that is meant for RS485 or has specified the impedance. Usually the datasheets do not contain enough parameters to calculate the impedance at the frequency you want.

If you are unsure, why even bother worrying about it. Always terminate the bus on both ends so you don't have to worry if it is needed or not.

Tolerance is not specified by the standard. Interfaces that use RS485 over 120 ohm cables define it as 120 ohms 5%. In fact, the termination is there so the cable length does not matter at all - when a cable is terminated at the characteristic impedance, it looks like an infinitely long transmission line.

You would have to know the signal edge rate to know if 200 m is long or not. It depends on the driver slew rate. Note that you don't have to think this at all, if you use termination.

115200 baud is do-able for 200 m. But you don't have to calculate the flight time or be worried about slew rate, if you terminate.