Ethernet NIC architecture: Function of loopback circuit
The SW 127.0.0.1 loopback tests the network stack but not the NIC.
The NIC loopback tests the network stack and the NIC card.
Cable loopback tests the network stack, the NIC and the connectors.


For the NIC card the loop back is a "mode" of operation, A properly designed NIC would deal with the collision detection appropriately, in full duplex, half duplex etc. situations.

I Think that your second item, "TX transmits to ..." is not right.  I'd say that in loop back the TX transmits only to the RX channel and NOT to the connector/wire as that would defeat the intent of loopback testing.  Likewise for your third item, the RX in loopback would only receive from the TX and not the wire also.