Now if the Shattered Flail is a heavy weapon then why is it that it can be used by ShadowHeart in one hand and her shield in the other? Yet you cant dual-wield it with another weapon makes no sense.
Yes, the problems are D&D rules and not all of them are well explained in EA.
If you want to attack with offhand, you have to respect some clauses, including both weapons with the "light" property (short swords and sickles, for example).
If the weapon does not have the "light" property it can be used in one hand and you can keep a shield in the other.
The "heavy" property only indicates that small creatures have disadvantage on attack rolls with it.
Instead, the "two-handed" property forces you to keep a weapon with two hands to attack with it.
I hope this can help you
https://roll20.net/compendium/dnd5e/Weapons#toc_2