A - Reread what I wrote; *you* can move with a line (meaning retreating, advancing, lunging, etc.). B - There's nothing supported in the rules that say you can't 'derobe' against a non search. If someone is literally in the process of an attack, and you derobe (clearly against a non-search, or your opponent wouldn't be attacking yet), then yes, your original line is removed and the new 'line' does not pre-exist the attack. You don't 'lose the line' because you're searching (you're not), you lose the line because you took it down. As long as the line (as interpreted by the ref) pre-exists the attack (as interpreted by the ref), it can score. That being said, if you derobe against nothing, re-establish your line (at the end of the derobement), and then your opponent begins his/her attack, you have a line again. But that'll be a bang bang call by the ref.