Don't cheat in this game.
We've seen that Konami will, in all likelihood, ban you for it, just looking back at how they reacted to people exploiting roaming duelists.
They will find out about what you're doing, and you will get banned for it.
for unlocking the mission, can I use for example, Paradox Brothers to duel on YGO realm, not the GX. Will it still count for the mmission in GX? Or am I strictly limited to using either jaden, chazz, or aster to unlock him? It's so hard since those three doesn't have LAB SKILL, nor LAST GAMBLE, CERB SKILL, I don't have any useful skills on them yet