That looks like a cool list
I've been looking for the cheapest way to get Harlies into a pan-Eldar list (for fluff purposes really) and hadn't considered the Heroes Path formation...
I'm fairly certain though that you can't take 2x Mask of Secrets. I thought that all relics were '1-per-army'. If that is the case, you could take the Armour of Misery on the Prince. It's got a similar leadership debuff, although doesn't give fearless (not really a problem with Coven Grotesques) and you're paying for armour bonuses that are redundant.
Still, I think the Shadowseer with Psychic Shriek would benefit from the -2 Ld debuff of the Mask more.