Is WoL the only one able to master/handle all the classes at once?

[Iscah]

Well she """fought""" in a level 70 battle wearing level 60 gear, so she must have made some progress beyond level 5.

It went about as well as a Lv60 player in a Lv70 battle would expect, too.

What is the canon explanation for that? Does the sheer power of the gear overwhelm the user and melt their face like they just looked into the Ark of the Covenant?

I figure maybe it's that whatever stat-boosting enchantments are in the clothes make them feel uncomfortable (if not outright dangerous as in the quote earlier) if you're not sufficiently trained to wear them.

Yshtola was a Miner in the beginning of ARR as well!

She dressed up as a miner at one point in post-ARR, but that's all.

You may be thinking of Minfilia who is mentioned a few times to have been a miner previously.