Here's the thing: would YOU go overseas if you were Bill Walker? I mean, I know I wouldn't. I'd force Ainge's hand. If Ainge doesn't want to pay him, he can renounce his rights, and I guarantee you some other team will ante up some dough to get Walker.
That's what I do if I were in his shoes and I would recommend him to do so I were in his agent's shoes. Force the issue and play in the NBA. Similar to Nichols last year.
I don't think Walker has any chance of forcing the Celtics hand... the Knicks were looking for people to make their team, they were a mess. They had a loaded roster, signed Nichols (non-guaranteed) and then they cut him.
In the Celtics, the message is clear... you sign with us, you get sent to the D-League. He's currently injured, the Celtics are in no hurry to make him sign a contract, they can retain his rights through the whole year. So no matter what situation unfolds, Walker is not going to play in the NBA this year, and there's little that he can do about it (unless he gets traded). If he gets traded, it won't be because he forced Danny into doing it, but because an opportunity arised that required Walker.
Walker, in the end, is better off just signing in Europe.