Suggestion on Input selector
It seems you are going about this the wrong way.  If TX and RX are connected, then you don't have TX and RX, you have a single signal.  Some common ways of dealing with this:

Use a line driver with output enable.  The higher level protocol then guarantees that only one node is driving the bus at the same time.  RS-485 is a example of this, in this case using a differential pair for the signal.

The bus passively floats to one level, and nodes only ever drive it to the other level.  IIC and CAN are examples of this, where IIC is single ended and CAN differential.

Each node drives and receives with a carefully controlled impedance.  If done right, two nodes can use the same line transmitting to the other simultaneously.  Old style analog telephone is a example of this.