this is the method i was referring to
I'm attaching a screen of the legendary dragonmother in the shop. It could be that, as you pointed out, that particular purchase gets double checked since it requires real cash, however the hero itself shouldn't be very hard to get with GG, i think. I say this because every other hero is available at legendary tier in the mercenary post and the dragonmother herself is available at rare and below. Its' just the legendary tier of that specific hero that won't show up at random like every other.
Anyway thanks for those tips, i'll try to dig the heroes structure