Simply put, because of the weight of intellect and spellpower, weapons with a higher iLvl are generally always better. Red values multistrike at .79, haste and .71, spellpower at .91, and mastery at .53 for every 1 point of intelligence. You can put together your own stat weights if you'd like, but expect to reach similar values, at least for the int:spellpower:multistrike ratio. Ultimately - you do you. The Xhul'horac staff isn't significantly worse than the Archimonde staff, and is definitely more accessible. If you want to be more specific: Edict of Argus (Archimonde) > Gavel of the Eredar (Archimonde) + Gibbering Madness (Gorefiend) > Voidcore Greatstaff (Xhul'horac); this most clearly shows that when it comes to weapons, iLvl is generally king.
Yes, it's pretty much a constant 6% haste. Regarding the 4piece, hard to say, unfortunately, as no one's had a chance to try it and the wording is awkward. My guess is it just has a 10% to take the healing value of a PoM proc, and heal someone else for the same (ie effective 10% buff to PoM).