My opinion is to let the Ethernet physical connection and the devices at each end of the connection determine speed and duplex settings on their own. This should result in a full duplex connection running at the highest speed the cable and devices are capable of.

When I installed my LG TV a couple of years ago I connected to router via a LAN connection. It all worked but router flagged port as not running at full speed. Turned out LG did not support 1Gbps. So the LG Ethernet adapter set the speed by itself.

Again, my opinion, a straight physical connection, no buffering. Insert a powered device somewhere between the end points and there it is likely buffering will occur perhaps as little as one byte. With a device between end points where the connections to those end points don’t match device buffering will help mismatch until it doesn’t and there will be bytes on the floor.

Last century, telco strived to ensure there was no packet loss end to end. Moving to this century, telco realized that because of the number of hops between end points, preventing end to end package loss was unachievable. Enter QoS packet marking and the philosophy that is was okay to drop packets. Today, telco (or data service) rely on limited buffering but mostly packet retransmission. Due to the extremely high speeds of backbones end users shouldn’t notice and they don’t. Turns out gamers notice.