You see, you state the impossibility to reroll as a fact, so let me show a case of my own:
Our maintank played
his warrior for a majority of this expansion, up till 7/10 mythic. Then decided to reroll
druid at the beginning of February. He leveled it up, farmed some dungeons and the original assumption was that he would still finish Nighthold on his warrior.
Around mid February he actually decided to reroll for the end of Nighthold instead. He did mythic EN and heroic NH on his druid to gear it up and on February 22nd recleared 7/10 mythic Nighthold and by now finished Elisande and Gul'dan progression with it.
Somehow "the impossible" was done in 3 weeks.