A partial answer. I saw a Vietnamese being deported on an EVA flight. She made a token struggle going down the stairs towards the gate waiting area. She was cuffed to one of them. I didn’t see how she was boarded.
A friend of mine (who is a purser) said she has refused to permit a deportee to board on safety grounds. She told me that ultimately the captain can always refuse to carry someone and would normally act on the advice of the purser. Normally she would expect someone to be travelling with the deportee if they had had to be escorted as far as the gate.