I don't know if this item will make into S2 or just for Beta. But when a necro equips AFTER pet is summoned the negative effect of the Myth have zero effect on the necromancer. So they can freely /switch myth and the negative resists/stats don't carry to the summoned minion. As for the Con makes sense that It wouldn't effect the servant but the negative resist in theory should as my character portrait shows the negative resist. The item debuff doesn't carry to the servant.
Item in Question
Stats before Equip (wearing Dragon Intellect Myth)
Resists/After Equip of Endless Dream
Like I said if it doesn't make S2 it's no issue, but if it does figured I'd do my part since it tech is a bug