Electronic – Electrical difference between Push pull and open drain

The question seems rather confused in several respects...

1) It is unclear whether you want to implement 3-state logic or an open-drain interconnection.

In the former, the driving device DOES use a push-pull output, driving 0 or 1 onto the bus. The other devices, meanwhile, must abstain from driving until some separate system signals that it is their turn.

In the latter, the bus is always pulled to '1' by a resistor, and any device may pull it to '0'. In this case, there is no harm done if several devices drive the bus simultaneously, though any messages may be corrupted.

2) You say you want a "floating" bus to indicate that the bus is free. In neither case is this normally possible (there is no logic primitive that can detect that a bus is floating). This is why in tri-state logic there must be another system (bus arbitration logic) to keep track of who has the bus, and give each device a turn.

Now as to the specific question of transmission gates : either of these systems can be trivially implemented using transmission gates, though there are other and sometimes better ways.

Tri-state logic can be implemented using a normal push-pull (totem-pole) output, and a transmission gate between the push-pull output and the bus. The bus arbiter simply switches the transmission gate on or off. It is often more economical to turn off both transistors in the push-pull output instead.

Open drain logic can be implemented with a transmission gate by simply connecting one side of it to ground, and the other to the bus. Now simply turn it on to pull the bus low.

Push-pull or open drain is relevant only for output ports. So for the SS1 pin, it does not matter.

However, to drive the TRIAC1 wire, given the schematic you gave, it is mandatory that you use push-pull. If you use open drain, either TRIAC1 is pulled to ground (with logical state 0), or unconnected (with state 1). In both cases, the optotriac LED wouldn't turn on and the triac wouldn't trigger. If you use push-pull, you can source current to the led (with state 1) so it's ok.

Now, there would be ways to make it work also with open-drain outputs, without using more components: if you connect the anode of the led to +3.3V and the cathode to the resistor and then to the TRIAC1 output of the MCU, you can use both push-pull or open-drain, as you wish. In both cases, you'd be able to turn the light on by setting the output to logical state 0 and turn it off by setting it to 1.