You are arguing due process. What others are saying is that the US is powerful enough to cause every other country on the planet to behave in a way that would allow due process to bring anyone in a participating country (so not Yemen or Syria) to a US prison. I dont think this is controversial.
If you are in a participating state and the US government wants you for any reason, they can set off a chain of events that will bring you to them. If you are in a non participating state they will just send the drones and it doesnt matter if you are a US citizen, a child, or a bystander.
So, Assange (and his supporters) are saying-- we dont want Assange to avoid justice, we want him to avoid parts of the justice system which may be bending under the influence of the US. The way they are expressing this is by themselves trying to bend parts of the system.
You have to remember, laws dont mean anything anymore, if they ever did. Its been 800 years since Magna Carta and we dont believe in its ideas anymore nor any of our other founding principals. You arent innocent until proven guilty if your guilt makes a good news story, you dont need to be present, you dont need to know who your accusers or judge is, you dont need representation by someone who understands the law, there doesnt need to be any proof you committed a crime and if its going to be too expensive to arrest you and hold your trial we will just blow you up remotely.
The Authorities have had years to question Assange in UK but clearly that is not what the bosses were telling them to do. Until, a few weeks ago that is. Maybe the pressure is subsiding and now justice can be pursued. Or maybe Assange will come down with a tragic case of radiation poisoning shortly after. Who knows.