OK.
"Charm does not equal love."
When the CHARM job ability is used, a long chain of PINK LOVE HEARTS goes from you to the mob. Also it is simple logic, if the person/mob loves you they will follow you blindly as a pet and they will die for you. They wouldn't die for you if they only quite liked you a little bit.
I played BST main job from Summer 2004 to late 2006, when I changed to PUP. During those 2+ years when I played BST every single day in the old hardmode charm everything solo-to-75 and solo-merits, I totally noticed the difference on charming mobs on Darksday. My LS was all BST, until PUP arrived and many of us changed to PUP. All the BST on my LS reported the same thing about Darksday.
When I didn't use my lightstaff, my charm failed way more often. I tried this while meleeing with CHR axes and trying to charm same time, it never had the same success rate as using lightstaff.
I used to arrive for LS events and I would be Weakened and level 74 lol. My LS friends would say stuff like "didn't you used to be level 75?" and we would all laugh, and they would look at my weakened level 74 @$$, and simply say "Darksday?" . And everyone knew they meant that I'd had multiple mis-charm deaths on Darksday.
Merrow are aquatic, and the merrow zone is an aqua zone. That is why it has water def. The resist charm is because that species use 'belly dance' charm. It is a specific item relating to a specific species and area.