As far as im concerned, when it comes to speed running and thus also challenge modes every trick in the book is allowed as long as it is not a pure exploit. Resetting instance to keep buffs, using invis pots, dying on purpose, all are fine. Hell if we were able to use heroism, reset and get the exhaustion cleared but not the buff we would do that too.
That said im not a fan of scarlet monastary, scholomance and niuzao where you can do rediculous times by clever use of game mechanics. Asuming you have a DK in your group Scholo and SM can both be done about 1-2 mins faster than with a regular group. I honestly dont think DKs should be allowed to use those abilities in Challenge Modes but on the other hand if they removed that now they would also have to reset the times.