What about the toons that have already completed the quest and didn't get a chest? Are the quests repeatable?

no, they aren't until next year. You still can kill the scarecrow king if you group with another player who is on the quest though

Just did the quest and got loot for level 70 and 88 - not sure where the 91+ stuff is. Wonder if it is worth a petition ?

@Panther

As explained on some other forum, this update means that 91+ players will recieve chests when killing The Scarecrow King. It does not mean they will recieve chests with T9-T10 gear.