Yes you are correct in your thoughts about the junction and that is why they gold plate the copper in expensive laser systems, the gold is softer and acts like thermal paste between the gold plated LD die and the copper of the heat sink.
And again you are right "O" rings are no way to take up the space inside of the host when a sloppy heat sink is made, really a correct heat sink for the host should be made.
If the heat sink is made correctly to begin with it will be within .001 to.0005" tolerance and fit in the host like your hand in a glove !
When you screw the bezel ring down it should grab the heat sink so it is held tight with a full metal to metal contact, there should be no insulating rubber inside the host in contact with the heat sink if the heat sink is made correctly !
This is the difference between mass produced $9.00 heat sinks like some make and custom heat sinks like I make
good you can get right now, The best takes a little longer and costs a little more