I could be wrong, but I thought I heard one of the commentators say that it wasn't legal to go back to the racing line after making a move. And this would make sense, because as the attacking driver lines up for his braking point, the defending car is off his line, so the attacking driver might brake a bit later. Then the defending driver suddenly goes back to the original line, and because the other car is braking later, he can't slow down in time, and you have another Melbourne 05 (I presume, haven't seen it properly).
The corner they were talking about though, Lesmo 1, Lewis was on the outside and had already given up when Michael moved the second time back to the racing line. Lewis was never going to run deep and run on the outside line in this corner as it would never have worked. He'd also visibly backed off once he'd realised that and Schumacher hence retook the preferred racing line once it was clear Lewis wasn't going to make a move stick.
This is why I refer to it as a "gray area" - no two corners or situations are the same. If we were talking about the exact same moves and lines but in a different corner, say, turn 1 at Malaysia where someone could make an outside line work, then yes, Schumacher would be in the wrong.
But because we are talking about Lesmo 1, a corner where outside overtaking is rare and most of the time impossible, it means Michael's "2nd move" is a fair move as its accepted that the attacking driver would never have tried to put their car there anyway.
If Lewis had braked later and gone for the outside pass, it would rely on Michael to not only give him a huge amount of space in the corner, but to effectively back out of the throttle and give up the place instantly, as he would not have been able to turn tightly enough to give the room and carry the speed round the corner to defend his position. He was only ever going to run wide back to the outside line on the exit as this is the natural line around the corner. Due to this inevitable outcome, Lewis would have had to back off to avoid collision.
Basically, if Lewis had attacked on the outside in Lesmo 1, it would have required Michael to completely give up the position in order to make the move work. Obviously he wouldn't have, so Lewis would have had to back out. So instead we have Lewis not even trying the move and Michael making his 2nd move in the braking zone to return to the better line through the corner.
In this kind of corner, you can only ever hope to try and distract a defending driver and try to pull off a "switchback" if they try to defend the inside line.