Weapon Damage only applies to the Weapon it is on.
that comparison thing in the lower left of the screen isn’t very accurate, and you are right to check the Stat Page instead.
I don’t know what could have increased the Skullshield DPS so much, based on your MH affixes. even if it was an Eternal Crimson Blade of the Elements, there shouldn’t have been that big a change.
this post helps explain how to get the most out of Equality Set, and how easy it is to make it ineffective.
for Crystals & Myth Stones at floor 100 and every 100th floor up to 500, you stop finding the lowest CS/MS for each 100th floor, but the Rare & Ultra Rare CS/MS become a little easier to find. so on floor 200+, you stop finding Calcite & Angelite, (and the 2 lowest Myth Stones), and some of the rarer CS & MS start dropping more often.
here is a post to help when you run into monsters that are hard to kill.