Hoo boy, this is gonna be hard to answer.
1: page 244 gives the defination of simultaneous attacks, mentioning resolving all damage and effects before resolving any of the target's special rules or effects.
2: Knights Exemplar Errants self-sac ability it says "when this model is disabled, chose a friendly non-disabled model within 3"
At this point we know that all the models get disabled at the same time, and they can't sac to another model who is disabled.
3: the order of damage is roll to hit, roll to injure, mark damage. If that model loses all it's boxes, it is then disabled, then it is boxed, then it is destroyed. So things can trigger inbetween.
So, the Mangler charges and catches 6 errants in a tight formation so they are all possible self-sac targets (just bear with me). He picks one rolls to hit and rolls damage that model is disabled, then continues down the line until all those attacks are resolved. All 6 become disabled, but Rhupert gave them Tough on them the turn before and Harby is looking at that saying to herself, "I can't let ALL those die!". So NOW, the Protectorate player can chose to Martyrdom or have them make Tough rolls OR self sacrifice in any order. So model A opts to make a Tough roll he fails and is still disabled, Harby can now martyrdom OR a model can self-sac. Model B is in a key spot and Harby opts to just save him and not have him knocked down, so martys him. Model C also fails its tough roll, and can now self-sacrifice to model B.
Once they Self-sac the models are removed as part of that ability. After that, any models still disabled are boxed, then it is destroyed and finally removed from the table (not to be confused with Removed from Play).