If RS485 transceivers are used and:
1. If the buss is idle (you pick the idle time) then only the
master is allowed to enable it's output transceiver.
2. A particular Slave's output transceiver is only enabled by
the slave receiving a message from the PC that has that
slave's address along with a request for a response from the
slave.
The slave would release the buss after the message to the
PC is complete.
The slave should have a "Computer Operating Properly"
Interrupt enabled so that a "stuck" slave would eventually
release the RS485 lines.

This is a 3 wire solution.
The +sig twisted with -sig covered with a ground shield.
The slave micro's ground must be isolated from grounds other
than the PC's in order to avoid ground loops introducing
noise, although the differential receivers will reject much of
the common mode noise injected by moderate levels of ground
loop currents.

RS485 spec. says that it can drive 4000 ft at 100K bits/sec with 32 transceivers on the line.

I think that this system would satisfy the requiremets of no buss contention & long line noise imunity.

I have enjoyed the forum a lot & am grateful for all the experts that spend their time helping others.

If I am in error or am not understanding the problem correctly I
apolgise.