So, the exception is that you are allowed to move twice as long as you go line-off-line. Max went off-line-off though and has faced absolutely no consequences. So from the actions taken after the incident (none) it's legal to move twice under braking, surely?
That's the way I read it, yes. International Sporting Code Appendix L that covers driver conduct in all FIA events, lists in Chapter IV, 2b that "more than once change of direction to defend a position is not permitted". Thus two moves are illegal.
In addition 2b states " Any driver moving back towards the racing line, having earlier defended his position off-line, should leave at least one car width between his own car and the edge of the track on the approach to the corner."
This specifically states that if you have defended your position off-line, and move back towards the racing line, space must be given. You can argue that this is the only way you can possibly do two moves, and keep it legal. Of course it does not mention anything about two moves, and it can be intended for situations where you are off line from the start of the straight (which often happens), but nothing like that is stated. But yes, as far as I can see, this is the sole possibility of a double move that can be argued to be legal.
This is my opinion: First of all it's really hard to blame the car that's in front. The normal rule is that the car that's behind has the responsibility to make a clean overtake. Now you will probably be like: "That's not fair. Verstappen moved in the braking area. If this would be legal no one would be able to pass." That's true. You can't move in the braking zone. You can change your line once as a defending car though outside of the braking zone. My opinion: Verstappen didn't change his line. The only thing he did was closing the inside to give Daniel no space so he would need to go to the outside. He didn't really change his line. You can see this on the Max's onboard. The most important thing to watch when looking at the footage is the steering wheel and especially downshifting. You can see when downshifting he's not steering. He's going straight. This means the argument: "Max moved under braking" can be thrown out of the window. Also when looking at his steering wheel before this you can see he isn't really moving a lot. I wouldn't call this changing his line. The outside has always been open so Ricciardo should have gone to the outside. He thought he tricked Max into the outside so he could go to the inside. But Max accepted Daniel going to the outside and stayed at his line. I think this is a racing accident. If you would need to blame anyone it would be Daniel. 60% Daniel, 40% Max.
However, like the Stewards have noted, it's not about erratic and potential dangerous driving (which is the rule that covers a situation like moving in the braking zone and creating danger). It's about making more than one move. I don't think there are any doubts that Verstappen moved his car to the right after Ricciardo first moved right. Then later moved back left, thus moved twice.
Also, as long as it is done in a safe manner,- predictable, moving under braking are now permitted. As long as it's not a second move and/or creating a potentially dangerous situation.
Regardless, the issues here are before the actual braking zone.