...I think it was Tom's unspoken qualification that the assault was impossible if one intended that the assailant would get away.

Possibly, but the dialogue between Tom & Michael would indicate that he feels the impossibility lies in getting near Roth:

TOM: That plane goes to Miami.

MICHAEL: That's where I want it met.

TOM: Mike that's impossible - they'll turn him over to the Internal Revenue, customs, and half the FBI.

MICHAEL: It's not impossible. Nothing's impossible.

TOM: I'd be like trying to kill the president - there's no way we can get to him.

MICHAEL: Tom, you know you surprise me. If anything in this life is certain, if history has taught us anything, it's that you can kill anybody...