-I'm not sure what happens to the bus when everyone is turned off (idle line state), but it's a good question/point
Two ways to know:
1) termination done as in 10 ways to bulletproof 485
2) use a (pin for pin compatible) transciever stated Fail safe, such as the max3080 series.

You state that you are stuck with the hardware, but replacing a chip with a pin compatible should be possible.

Erik